Skip to main content

Full text of "NASA Technical Reports Server (NTRS) 19900004096: The Voyager Neptune travel guide"

See other formats

JPL Publication 89-24 

/A/- /% 

o r7 

The Voyager Neptune 
Travel Guide 


C 2 
►h > 


m > 


^ ;» 

c- I 
-o H-* 
r- cd 

v 0 s 


r\j a* 

>o ^ 


*U -H 












o m 

O *H 

n 7* 

fsj < 

isj m 

> r- 

° C ^ 

June 1, 1989 J$ - ? 

o a t- 

»— WD Ud 

f\JASA ~ 

National Aeronautics and 
Space Administration 

Jet Propulsion Laboratory 

California Institute of Technology 
Pasadena, California 

Voyager 2 approaches the sunlit hemisphere of the gas giant Neptune. 
In this computer-simulated view, one hour before closest approach on the 
evening of August 24 (PDT), 1989, we can see Neptune's ring arcs, believed 
to exist as a result of ground-observed stellar occultations. Just off the 
southern limb of Neptune (at about 7 o'clock), somewhat larger than the 
nearby star dots, we see the enigmatic moon Triton, which Voyager 2 will 
dive past six hours from now. 

Cover: Looking back at Voyager 2 and the Neptune system two hours after Triton 
closest approach. (Painting by artist Don Davis.) 



Page intentionally left blank 

JPL Publication 89-24 

The Voyager Neptune 
Travel Guide 

Voyager Mission Planning Office Staff 

Charles Kohlhase 

June 1, 1989 


National Aeronautics and 
Space Administration 

Jet Propulsion Laboratory 

California Institute of Technology 
Pasadena, California 

The research described in this publication was carried out by the Jet 
Propulsion Laboratory, California Institute of Technology, under a contract 
with the National Aeronautics and Space Administration. 

Reference herein to any specific commercial product, process, or service by 
trade name, trademark, manufacturer, or otherwise, does not constitute or 
imply its endorsement by the United States Government or the Jet Propul- 
sion Laboratory, California Institute of Technology. 


Voyager Neptune Travel Guide 

Contents Page 


Voyager’s Past 3 

Anticipating Neptune 4 

Lights, Camera, FLIP 7 


A Glorious Construct of the Mind 9 

What We Know 13 

Rewriting the Book 17 


Planning 21 

Sequencing 24 

Flight Operations 25 

Commanding 26 

Receiving Data 28 

The Results 30 


Bus 32 

High Gain Antenna 32 

Spacecraft Attitude Control 34 

Spacecraft Manuevers 34 

Scan Platform Pointing 35 

Power Subsystem 36 

Data Storage Subsystem 38 

Spacecraft Receiver 39 

Computer Command Subsystem 40 

Flight Data Subsystem 42 

Science Instruments 43 



Imaging Science Subsystem 45 

Infrared Interferometer Spectrometer and Radiometer 47 

Ultraviolet Spectrometer 48 

Photopolarimeter Subsystem 50 

Radio Science Subsystem 51 

Fields, Waves, and Particles Experiments 53 

Planetary Radio Astronomy 53 

Plasma Wave Subsystem 54 

Magnetometer 54 

Particle Detectors 55 

Plasma Subsystem 55 

Low Energy Charged Particle and Cosmic Ray Subsystem 55 

Sensor Engineering Characteristics 56 

The Physics of the Optical Target Body Instruments 56 


Getting Our Bearings 63 

Aiming for Neptune 66 

Setting Up for the Encounter 71 

Striving for Perfection 72 

Critical Late Activities 74 

Practice Helps to Make Perfect, Too 77 

How the Bits Flow 78 

Neptune at Last! 80 

Observatory Phase 82 

Far-Encounter Phase 86 

Near-Encounter Phase 90 

Post-Encounter Phase 100 


A Change in Attitude 103 

Real Applications 105 

Gaining Speed Along the Way 106 

Diving for Triton 107 

The Solar System is Ours 108 


Past Skirmishes: The Aches and Pains of Voyager 2 110 

The Goblins and their Mischief 112 

Taking the Plunge 113 

Outwitting the Goblins 116 


Maintaining a Strong Signal 123 

Discarding Unnecessary Picture Data 126 

More Accuracy for Fewer Bits 126 

Taking Good Pictures in Feeble Light Levels 127 

Diagnosing the Health of the Actuators 131 

Faster Response from the “Old” Robot 131 

Big Changes in the Deep Space Network 132 

The Bottom Line 133 


Overall Mission 135 

Voyager Spacecraft 136 

Navigation 139 

Science 140 

The Future 141 


The Grand Tour 143 

The Great Escape 144 

Voyager 2 at Neptune 146 

Key Events, Distances, and Speeds 147 


The Voyager Interstellar Mission 151 

Fields and Particles Investigations During the VIM 151 

Ultraviolet Astronomy During the VIM 153 

Spacecraft Lifetime 154 

To the Stars 155 


Jupiter 169 

Jupiter’s Atmosphere 171 

Jupiter’s Rings 173 

Jupiter’s Moons 174 

Jupiter’s Magnetosphere 178 

Saturn 179 

Saturn’s Atmosphere 180 

Saturn’s Rings 180 

Saturn’s Moons 183 

Saturn’s Magnetosphere 186 

Uranus 187 

Uranus’ Atmosphere 188 

Uranus’ Rings 190 

Uranus’ Moons 192 

Uranus’ Magnetosphere 196 

Cruise Science Results 198 

Ultraviolet Observations 198 

Imaging Science 200 

Fields and Particles Research 200 


Voyager Personnel 203 

Those Known Widely 207 

15. WHO’S ON FIRST? 211 


Magellan 222 

Galileo 224 

Wide-Field/Planetary Camera — Hubble Space Telescope 226 

Ulysses 228 

Topex/Poseidon 230 

Mars Observer 232 

Comet Rendezvous Asteroid Flyby 234 

Cassini 236 

Earth Observing System 238 

Mars Rover Sample Return 240 

Mission Concepts 242 





20. INDEX 267 

21. MANY THANKS 275 

22. HIP POCKET Inside Back Cover 


This publication describes, in simple language and with numerous 
illustrations, the epic Voyager mission to explore the giant outer planets of 
our solar system. Scientific highlights include interplanetary cruise, Jupi- 
ter, Saturn, Uranus, and their vast satellite and ring systems. Detailed 
plans are provided for the August 1989 Neptune encounter and subsequent 
interstellar journey to reach the heliopause. As background, the elements 
of an unmanned space mission are explained, with emphasis on the capabili- 
ties of the spacecraft and the scientific sensors. 

Other topics include the Voyager Grand Tour trajectory design, deep- 
space navigation, and gravity-assist concepts. The Neptune flyby is ani- 
mated through the use of computer-generated, flip-page movie frames that 
appear in the corners of the publication. Useful historical information is also 
presented, including remarkable or gee-whiz facts associated with the 
Voyager mission. Finally, short summaries are provided to describe the 
major objectives and schedules for several exciting space missions planned 
for the remainder of the 20th century. 


Let your soul stand cool and composed before a million 

Walt Whitman 


Congratulations! You are now the proud owner of a Voyager Neptune 
Travel Guide, hereinafter simply called the Guide. Its purpose is to explain 
in simple language, and with numerous illustrations, the Voyager-2 plans 
to examine Neptune and its moons, possible ring arcs, particles, and fields. 
A major attraction will be Neptune’s large and unusual moon Triton. The 
Guide will also contain a variety of interesting facts about the Voyager 
mission, both past and future. 

Before jumping into the particulars of the Neptune mission, let’s briefly 
review the basic elements of an unmanned space mission. These elements 
are shown in Figure 1-1. You must, of course, have a Spacecraft capable of 
carrying a variety of sensors to the destination in order to conduct the 
Science you have in mind. The spacecraft cannot escape from Earth’s 
gravity well without the help of a Launch Vehicle, either an expendable set 
of rocket stages or a reusable Space Transportation System, or Shuttle, with 
a high-energy upper stage. 

No launch vehicle or spacecraft has an error-free guidance system, and 
so the process of Navigation is necessary to deliver the spacecraft to a 
precise location at the destination. As shown in Figure 1-2, the navigation 
process uses range (distance) and doppler (range rate) measurements from 
huge tracking antennas to estimate the spacecraft location to an accuracy of 
1000 to 3000 km (620 to 1860 mi). As the spacecraft nears the destination, 
it takes pictures of natural satellites against a star background (a technique 
called optical navigation) to estimate its position to within 100 km (62 mi). 
If the spacecraft’s flight path is off course, mission controllers send com- 
mands that cause the spacecraft to use small thrusters to correct its course. 

As you have guessed by now, Voyager needs a lot of support from Earth 
Base. Voyager can cruise happily along, locked onto the Sun and a guide 
star, even using onboard fault protection logic to react to problems, but it still 
needs to hear from Earth regarding its activity plan. 

A group of scientists decide upon an observation they would like 
Voyager to make. Flight team personnel from areas such as mission 
planning, science support, spacecraft engineering, flight operations, and 
sequence implementation, schedule and design the observation into a 


Figure 1-1. These are the five basic elements of an unmanned space mission. Earth 
Base is composed of a large complex of people , computers , communication lines , 
and tracking antennas. A manned space mission has a sixth element , the human 
crew for whom life support systems are required. 


Red Spot 


Conflict-Free Timeline 

I* r ’ 

| Planning 




CMD Load 

Target Pointing 
Command File 

Figure 1-3. Many steps are necessary to develop activity sequences that Voyager 
will eventually execute. 

master activity timeline. As shown in Figure 1-3, several steps are taken 
before Voyager finally carries out these instructions from Earth. Since 
Voyager has its own internal clock, desired activities can be loaded into its 
computers many days before they are to be executed. Each set of activities 
is termed a command load. 

Voyager's Past 

The Voyager mission has had quite a past. As shown in Figure 1-4, 
the two spacefaring robots were launched from Earth in 1977, bound for the 
giant planets of the outer solar system. These amazing machines are like 
distant extensions of the human sensory organs, having already exposed the 
once-secret lives of some four dozen worlds. Like remote tourists in never- 
never land, they have snapped pictures to reveal Saturn’s dazzling necklace 
of 10,000 strands. Millions of ice particles and car-sized bergs race along 
each of the million-kilometer-long strands, with the traffic flow orchestrated 
by the combined gravitational tugs of Saturn, a retinue of moons and 
moonlets, and even the mutual interactions among neighboring ring par- 



Figure 1-4. Though not discernible in this view , Voyager 1 was deflected upwards 
by its pass beneath Saturn. Voyager 2 remains near the ecliptic plane until its dive 
over Neptune deflects its path sharply downward, below the ecliptic plane. 
Accelerated by gravity assist, both Voyagers will cross the orbits of the outermost 
known planets by the turn of the decade, racing onward to escape from the solar 

The Voyagers have shown us the erupting volcanoes of golden Io, the 
colorful and dynamic atmosphere of gargantuan Jupiter and its centuries- 
old Great Red Spot, the smooth water-ice surface of Europa that may hide an 
underground ocean, the strange world of Titan with its dense atmosphere 
and variety of hydrocarbons that slowly fall upon strange seas of ethane and 
methane, the small moon Mimas that was nearly destroyed by an ancient 
collision, the remote realm of tilted Uranus and its remarkable moon 
Miranda, and the many other wonders that have expanded the dimensions 
of our knowledge. 

Anticipating Neptune 

Can Neptune, discovered in 1846 at the Berlin Observatory (using 
mathematical predictions), possibly provide a level of excitement and wealth 
of new discoveries even close to those of the Jupiter, Saturn, and Uranus en- 
counters? At first, the aquamarine gas giant Neptune appears to be Uranus’ 


fraternal twin . . . but size and color alone tell only part of the story. Though 
Neptune is much farther from the Sun than Uranus is, its overall tempera- 
ture is roughly the same, suggesting to scientists that Neptune has an 
internal heat source of its own, perhaps similar to those of Jupiter and 
Saturn. Each of the four seasons lasts more than 40 years, a period 
comparable to that of a human's entire working career from graduation to 

Neptune has two known moons. Nereid, only 800 km (500 mi) in 
diameter, orbits so far from its planetary overlord that nearly one Earth year 
must pass before it can complete one lap around Neptune. Triton, on the 
other hand, is roughly the size of Earth’s moon and laps the planet every six 
days in a direction opposite to the planet’s spin. Ground-based observations 
of Triton indicate that it may have a thin atmosphere covering an icy surface 
with shallow pools of nitrogen, possibly liquid, but more probably cold 
enough to be solid like vast slabs of glass. Though not a sure thing, scientists 
are betting that Voyager’s cameras will be able to photograph Triton’s 
surface . . . unlike the circumstances at Saturn’s haze-enshrouded moon 

Humans find planetary rings beautiful, as borne out by the public’s awe 
during the three previous encounters with gas giants. As if to spice matters 
up a bit, ground-based stellar occultation measurements seem to be saying 
that Neptune also has rings . . . but only in the form of many short arcs that 
do not connect like a necklace. The Voyager scientists are very excited about 
such a possibility, and different theories are being passed about to explain 
such an unusual situation. 

Aside from the above scientific tidbits, which will be explored more 
completely in the next chapter, there should be an air of drama during 
execution of the encounter sequences. Voyager’s close dive over the northern 
polar region of Neptune will provide only slight clearance above the outer- 
most ring-arc region and the detectable atmosphere, and the radiation 
effects from particles trapped by the magnetic field cannot be disregarded. 
The round-trip communication time will be 8.2 hours; we will be slewing the 
same scan platform that became stuck for a period following the Saturn 
encounter; we will be using an onboard computer to compress the number of 
picture bits sent back to Earth; and, we will be programming Voyager 2 to 
perform several maneuvers to allow the cameras to take sharper images. 
The navigation challenges will also be worrisome at such a great distance 
from home. The bottom line? There should be plenty of excitement, as well 
as a few surprises, during the upcoming encounter. 


The keys to the mysterious kingdom of Neptune lie just within our 
reach as Voyager 2 draws closer. The purpose of this Guide is to tell a piece 
of that story, thereby kindling our human quest to understand worlds 
beyond Earth that comprise a small region of the larger cosmos. 

Notice: The information in this Guide was accurate at the time of publication, but may change 
by small amounts as time passes. Of primary relevance are some of the Neptune system 
physical characteristics, a few observational designs, and certain precisely quoted miss 
distances and event times. 


Lights , Camera, FLIP . . . 

The flip-page movie in this Guide is your own animated memento of 
Voyager’s historic swing past Neptune and Triton. The 133 frames cover a 
time period from 3.5 hours before to 7.5 hours after closest approach to 
Neptune. Watching the action through a 50° field of view, we pick up the 
spacecraft as it approaches the planet from slightly below the ring arcs, and 
ride along behind it as Voyager 2 sweeps up through the ring-arc plane, 
comes within 4850 km of Neptune's cloud deck, and then passes through 
Neptune’s shadow. The suspected ring arcs are believed to move in the 
circular orbits shown outside of Neptune in its equatorial plane. According 
to some theories, a small 200-km-diameter moon (as yet undiscovered) orbits 
just beyond the orbit of the outermost ring arcs, determining the number of 
arcs in each of several interior orbits. 

The gravity field of Neptune bends the flight path sharply south, 
below the ecliptic plane. Soon after closest approach to Neptune, we reduce 
our field of view to 20° as the spacecraft turns its attention to the moon 
Triton, and passes 40,000 km from Triton’s center. The hatching next to the 
terminator denotes the body hemisphere in shadow. Its planetary encoun- 
ters completed, Voyager 2 sails on towards interstellar space, leaving us with 
a parting glance back at the crescents of Neptune and Triton. 

For your reference, the movie frames contain time and distance infor- 
mation. Both time from Neptune closest approach and “calendar time” are 
shown. The latter refers to GMT (Greenwich Mean Time) in SCT (spacecraft 
time). If you want PDT (Pacific Daylight Time), subtract seven hours. If you 
want ERT (Earth-received time), add 4 hours 6 minutes. The distance shown 
is measured from the viewer (near the spacecraft) to the center of either 
Neptune or Triton. 

The wire-frame images were designed by the Voyager Mission Plan- 
ning Office using the VAX-based SPACE software created by the JPL 
Computer Graphics Laboratory. 


TIME: -0 Days, 3:30:00 
1989/08/25 0:30 GMT (SCT) 



250012 KM 

Astronomy compels the soul to look upwards and leads us 
from this world to another. 



The final “stop” on our Grand Tour of the outer solar system is the 
planet Neptune. It is a stop in name only, for we do not really stop. In fact, 
as we get closer to Neptune, we speed up. It took twelve years to make the 
journey and we are very close to Neptune for but a few short days. During 
this period we will take the most detailed photographs of the planet and the 
only close photographs of its moons and possible ring system likely to be seen 
during our lifetimes. 

A Glorious Construct Of The Mind 

As you may recall, Sir William Herschel discovered Uranus from his 
backyard in 1781. Uranus was the first planet discovered since ancient 
times, causing great excitement in the world of astronomy. Anyone who 
could acquire the use of a telescope turned to observing the new planet. 
Scientists began cataloguingits position with time, to predict where it would 
be in the future. As the 18th century closed and the 19th century opened, 
astronomers began to have a small amount of trouble predicting the future 
location of Uranus. The planet simply did not appear in the part of the sky 
where it was supposed to be. 

Isaac Newton’s first law of motion states that once an object starts 
moving it keeps moving in exactly the same way unless a new force acts to 
change its motion. A fundamental law of matter (Isaac Newton’s Law of 
Universal Gravitation) is that all bodies pull on each other. This pull is the 
only significant force that acts on planets as they orbit about the Sun. The 
largest pull is, of course, provided by the Sun itself. But each planet exerts 
a small pull on every other planet. All of these pulls must be taken into 
account when predicting the future location of any planet. 

By the early 1800s, the difference between where Uranus was predicted 
to be, taking into account the other known planets, and where it actually 
appeared was getting to be quite noticeable. As early as 1824, Friedrich 
Wilhelm Bessel suggested that a new planet must be pulling on Uranus, 
causing the unpredicted motion. 

At least two young mathematicians, working alone and unbeknown to 
each other, attempted to predict the size and location of the new planet that 


Figure 2-1. Probably the first person to suggest that the irregularities in 
Uranus' orbit were caused by a new, more distant planet was the German 
mathematician Friedrich Wilhelm Bessel. (A.H. Batten. Resolute and 
Undertaking Characters: The Lives of Wilhelm and Otto Struve . 1988. Permission 
granted by Kluwer Academic Publishers, Dordrecht, Holland.) 

TIME: -0 Days, 325:00 
1989/08/25 0:35 GMT (SCT) 

must be pulling on Uranus. In England, John Couch Adams completed the 
calculations first, in 1845. He privately informed the English Astronomer 
Royal, George Airy, that if one was to look in a certain 
place at a certain time one would discover a new 
planet. Airy chose to disregard the prediction and did 
not make the observation. Subsequently, Airy did 
send Adams’ calculations to James Challis, Plumian 
professor of astronomy and director of the Cambridge 
Observatory. Ironically, Challis recorded the new 




planet twice without realizing his success. Along this vein, more than fifty 
years earlier, Joseph Lalande recorded the new planet twice over three 
nights . . . but attributed the slightly different positions of this find to obser- 
vational error! 

Meanwhile, in France, Urbain Jean Joseph Le Verrier completed his 
own calculations the following year. He turned his results over to both Airy 
and the French Academy of Sciences in published form, with a prediction on 
where and when to look to discover the new planet. His prediction was within 
one degree of Adams’ earlier independent prediction. 

The same fate befell his work as befell Adams' results: no observers 
used the predictions to look for a new planet. Finally, almost in desperation, 

Figure 2-2. The first person to calculate the location of Neptune was the 
English mathematician John Couch Adams. Unfortunately, Adams did not 
publish his work right away , and the calculations of another were used to 
discover the new planet. (Robert Ball. Great Astronomers . London , 1895.) 



black and white photograph 

Figure 2-3. The French mathematician Urbain Jean 
Joseph Le Verrier was the second person to determine 
the location of Neptune, but the first to publish his 
results. When no one would listen to his prediction, 
he mailed a request to a young astronomer at the 
Berlin Observatory, asking for a search. ( Illustrated 
London News. February 2, 1847.) 

Le Verrier sent his results to the unknown young German astronomer 
Johann Qalle. The night of the day he received Le Vender’s prediction, 
September 23, 1846, Galle and a graduate assistant, Heinrich d’Arrest, with 
the aid of recently constructed star maps, sighted an unknown “star” in less 
than one hour and within one degree of where Le Verrier said Neptune would 
be. When, the following night, the new star exhibited a disk and had moved, 
the discovery was confirmed. 

Perhaps the greatest intellectual accomplish- 
ment possible is to predict the existence of a phenome- 
non of nature before it is ever observed. Adams and Le 
Verrier independently accomplished this feat by cor- 
rectly predicting the existence and location of Nep- 
tune before it had ever been observed. 

TIME: -0 Days. 3:20:00 
1989/08/25 0:40 GMT (SCT) 


239099 KM 


Figure 2-4. The night of the very day that Johann Galle received Le Verrier’s request, 
he and a graduate assistant, Heinrich d’ Arrest, sighted Neptune. This is a copy of 
the actual star chart used by Galle and d’ Arrest to discover the new planet. The 
square is the position of Neptune predicted byLe Verrier. The circle is the position 
where Neptune was first observed. (Hora XXI of Berlin Academy's Star Atlas, annotated 
by f.G. Galle. Courtesy Archenhold-Sternwarte, Berlin, East Germany.) 

What We Know 

One hundred and forty-three years have elapsed since the first obser- 
vation of Neptune. In that time we have acquired only the most basic 
knowledge about Neptune and its environs. We know how far it is from the 
Sun (30 astronomical units, or AU), its basic color (blue), size (57 times the 
volume of Earth), atmospheric composition (mostly hydrogen, helium, and 
methane), mass (17 times the Earth), effective temperature (about 60°C 
above absolute zero), the length of its year (165 years) and day (nearly 18 
hours), and that it has at least two moons (Triton and Nereid). We know 
precious little else about the place. 

Neptune is a gas giant, literally an immense ball of various gases that 
become denser as one goes deeper, with no solid surface as we know it. 
Jupiter, Saturn, and Uranus are all gas giants. When one looks at the 
photograph of a gas giant, one is looking at the top of the atmosphere and, 
when warm enough, the cloud tops just below. Uranus appeared bluish 
green in color. 

Neptune is expected to look much the same way, and for exactly the 
same reason. The blue is produced by sunlight (of all colors) being robbed 
(absorbed) of its red and orange colors by methane. The remaining blues, and 




Figure 2-5. In these Voyager images taken in April and May 1989 , Neptune shows 
cloud-type features much earlier than Uranus did. At a resolution nearly five times 
better than Earth-based the hints of “gaseous topography” are tantalizing. 

to a lesser extent greens, are reflected back to the viewer. Earth-based 
observations, as well as those by Voyager (see Figure 2-5), have indicated 
that Neptune has large cloud features. 

Neptune’s equator is tilted with respect to its orbit about the Sun by 
almost 29 degrees. This is slightly more than the 23.5 degrees of the Earth’s 
tilt, and produces the same effect: seasons. Voyager 2 will arrive during 
winter in Neptune’s northern hemisphere. Temperatures range from a mini- 
mum of about 50°C above absolute zero some 50 km above the cloud tops to 
several hundred degrees warmer as one goes deeper into the gases. Neptune 
gives off more than twice as much heat as it receives from the Sun. 

Until 1988, we had no evidence that Neptune had a magnetic field. 
However, recent Earth-based observations have detected what may be 
synchrotron radiation near Neptune, an indication that this planet, too, has 
a magnetic field. Estimates of the surface field strength range from 1/2 to 1 
gauss-about equal to or twice that of Earth’s magnetic field strength at the 

English astronomer William Lassell first observed Neptune’s large 

1989/08/25 0:45 GMT (SCT) 

TIME: -0 Days. 3:15:00 

moon Triton a mere two-and-one-half weeks after 
Neptune itself was first observed. Unfortunately, a 
solar conjunction prevented the discovery from being 
confirmed until July 1847. Only one other Neptunian 
moon has been discovered to date. The small moon 
Nereid was first observed by American astronomer 
Gerard Kuiper in 1949, only forty years ago. 



233633 KM 

Figure 2-6. Neptune is really a very simple gas giant. It probably has a thin outer 
atmosphere of hydrogen, helium, and methane gases. The interior is probably a 
mixture of rocks, ices , and liquids, with more rock the closer one gets to the center . 

Triton may very well prove to be one of the most interesting places in 
the solar system. It certainly has the credentials. The moon orbits Neptune 
in a retrograde or backwards direction, opposite to the planet’s rotation on 
its own axis. Triton’s orbit is also tilted with respect to Neptune’s equator 
by some 20 degrees. Various scientists (Issei Yamamoto in 1934, Raymond 
Lyttleton in 1936, and Gerard Kuiper in 1956) have proposed that some 
cataclysmic event in the past flung Pluto into its own tilted orbit about the 
Sun, and wrenched Triton into its current backwards tilted orbit about 
Neptune. However, the discovery of Pluto’s moon Charon has cast doubt on 
this hypothesis. 


Figure 2-7. Neptune has two known moons: the largish Triton and the smallish 
Nereid. The Voyagers discovered three new moons each at Jupiter and at Saturn , 
and ten new moons at Uranus. Small moons about distant planets are hard to detect 
from Earth. Thus , the potential for Voyager 2 to discover new moons at Neptune 
is high. 

Triton takes just under 6 days to complete one orbit, and this period is 
steadily (but very slowly) decreasing, as Triton spirals closer to Neptune as 
a result of tidal interactions. It has been estimated that within one hundred 
million to one billion years Triton will spiral in close enough to Neptune to 
break up and become a new set of rings. 

Triton’s size is quite uncertain, ranging from smaller than Earth’s 
moon to possibly as large as the planet Mercury. 

It gets more bizarre: Triton appears to have a thin atmosphere. Earth- 
based observations show absorption due to methane frost and/or gas. How 
much methane may be present is unknown, and other gases such as nitrogen 
may also be present. The surface temperature is near the triple point tem- 
perature of nitrogen. Earth-bound readers of this 
Guide will appreciate the significance of a triple point. 
Both the pressure and temperature of the surface of 
the Earth are near the triple point of water, allowing 
solid, liquid, and vaporous water to coexist. Although 
Triton’s atmospheric pressure is expected to be well 
below nitrogen’s triple point pressure, it is possible 

TIME: -O Days, 3:10:00 
1989/08/25 0:50 GMT (SCT) 


228160 KM 


that shallow pools of liquid nitrogen or, more probably, glaciers of frozen 
nitrogen may be present on Triton! 

The state of knowledge about Nereid makes the state of knowledge 
about Triton look encyclopedic. Nereid is known to orbit about Neptune in 
a normal manner (i.e. , in the same direction that the planet rotates on its own 
axis), albeit sixteen times farther away from Neptune than Triton is, and to 
take almost an Earth year to do it (see Figure 11-3). The uncertainty in 
Nereid’s size is such that the best estimate recently increased by 60 percent! 
This moon is thought to be an ice ball, with perhaps some rocky material dis- 
tributed throughout. Some astronomers report that Nereid’s brightness has 
been observed to change by a factor of four over a period of 2-1/4 days. If true, 
such variations could be caused by apparent size or surface brightness 
changes as the satellite rotates. 

All of the preceding is totally straightforward compared to the state of 
affairs on the Neptunian ring system. Until this decade there was absolutely 
no indication that Neptune had rings. In the past nine years astronomers 
have observed numerous stars as Neptune passed in front of (occulted) them. 
Out of one hundred and ten such occasions, eight times a mysterious event 
occurred. The starlight dimmed for a short period of time either before or 
after the star was completely occulted by the planet. 

The dimming of starlight from an occultation usually indicates the 
presence of a ring system. However, whenever planetary rings have been 
detected by stellar occultations, the ring occultations always occurred 
symmetrically both before and after the planet occultation. So what are we 
to assume? Because the starlight only dimmed, but did not totally vanish, 
tenuous ring material is a more likely culprit than a solid moon. 

In summary, only about seven percent of Neptune stellar occultations 
have produced “ring” occultations, and no “ring” occultation has been 
observed to occur both before and after the planetary occultation. The 
leading hypothesis to explain these bizarre events is that there is a set of at 
least three partially filled orbits of ring material (called ring arcs) encircling 
Neptune. About seven percent of the circumference of each ring-arc set is 
assumed to be filled with ice and rocks. It must be strongly emphasized that 
the “ring arc” explanation is the current best guess. There may well not be 
rings at Neptune. 

Rewriting The Book 

Voyager carries sensors for eleven scientific investigations. You will 
learn in depth about Voyager’s sensors and investigations in Chapter 5. Suf- 
fice it to say that Voyager “rewrites the book” by observing, both from a 


Figure 2-8. The large moon Triton , with an estimated diameter of 3000 km (1860 
miles), may prove to be the show-stopper at Neptune. About the size of Earth's 
moon, it may have a thin atmosphere of methane and perhaps nitrogen, with very 
shallow pools of Uquid or, more probably, frozen nitrogen on its surface. Its 
orangish hue, like that of Saturn's moon Titan, suggests that sunlight may be 
breaking down the methane and creating a variety of 
organic particles in the form of hydrocarbon-based aerosols. 
(Courtesy of the artist Ron Miller) 



TIME: -0 Days, 3:05:00 
1989/08/25 0:55 GMT (SCT) 


222680 KM 


Ecliptic North 

Figure 2-9. One hundred and ten attempts have been made to observe stars as they 
appear to pass behind Neptune. Eight times the starlight has dimmed , either before 
or after the star was occulted by the planet. The locations of the first six of these 
eight mysterious events are plotted in this plane-of-the-sky map of Neptune. The 
remaining two events are still being analyzed for precise locations. The current 
leading explanation involves three sets of incomplete rings , called ring arcs. 

distance and up close. The closer the spacecraft gets to its target, in general 
the better all of its sensors work. Voyager will make its most important meas- 
urements when it is closest to Neptune, Triton, Nereid, and the hypothetical 
ring arcs. 

For the Neptune Encounter, Voyager has three major scientific objec- 
tives: to help us learn as much as possible about Neptune, Triton, and 
Neptune’s magnetic field. On a more detailed basis these general objectives 
expand to: 

(a) Accurately determine Neptune’s basic characteristics: color, cloud 
features, size, mass, density, composition, temperature, temperature 
variations, heat balance, wind speeds, and rotation rate. 

(b) Determine Triton’s basic characteristics: color, surface features, size, 
mass, density, composition, temperature, rotation rate, if there is an 
atmosphere and, if so, its temperature, pressure, and composition. 

(c) Search for new moons, and characterize as many as possible. 

(d) Search for rings (or ring arcs), and characterize as many as possible. 

(e) Characterize Neptune’s magnetic field strength, center location and 
orientation, and the structure and composition of any charged particles 
within the resulting magnetosphere. 


(f) Search for other planetary phenomena, such as lightning, auroras 
(“Northern Lights”), and radio emissions. 

(g) Accurately determine the Neptunian system’s basic characteristics: 
position in time and rotational pole orientation. 

(h) Determine Nereid’s basic characteristics: color, size, and shape. 

Voyager is a robot, and must be told exactly what to do. Lists of such 
instructions are called software programs, or in Voyager parlance Computer 
Command Subsystem (CCS) loads. A CCS load consists of many sub- 
programs called links. Each link contains the instructions to accomplish a 
specific science observation or engineering support goal. For example, the 
link that takes the highest resolution photographs of Nereid is called 

The science links have a well-defined naming convention. The first 
letter designates the sensor (P = Photopolarimeter Subsystem, R = InfraRed 
Interferometer Spectrometer and Radiometer, U = Ultraviolet Spectrome- 
ter, V = Imaging Science Subsystem or TV, X = Radio Science Subsystem S/ 
X-band, A = Planetary Radio Astronomy, W = Plasma Wave Subsystem, and 
F = Fields and Particles [Low-Energy Charged Particle, Cosmic Ray Subsys- 
tem, Plasma Subsystem, or Magnetometer]). First letters of link names can 
also refer to engineering or navigation activities. 

The second letter in the link name designates the target body. (P = 
Planet [Neptune], T = Triton, N = Nereid, S = System, R = Ring arcs, H = 
Helios or Sun, X = unknown satellite, and C = Calibration or Configuration). 
The remaining letters and numbers in each link name provide a shorthand 
description of the observation. Chapter 6, Encounter Highlights, contains a 
summary of the times and goals of the most important science links. 

So where do we stand in our knowledge about Neptune? We are not sure 
if it has rings. We have no idea if it has lightning. The colors and sizes of 
Triton and Nereid are not well known. It is thought that Triton has an 
atmosphere of methane and perhaps nitrogen, but we do not know its 
thickness or whether the surface can be seen. Will the surface, if seen, reveal 
shallow pools of frozen or liquid nitrogen? It must be kept in mind that 
Nereid and Triton have never appeared in any tele- 

TIME:-0 Days. 3:00:00 _ . . ^ 

1989/08/25 i :oo gmt (sct) scope as any more than pinpoints oi light. Jiiven 

Neptune has never appeared as more than a fuzzy 
ball. The answers to the above questions, and many 
more, will hopefully be revealed during the summer 
of 1 989 as Voyager 2 provides our first close encounter 
with Neptune. 


TO NEPTUNE: 217193 KM 

Man alone is the architect of his destiny . 

William fames 


When you are asked to think about space missions to the planets, you 
will probably think about stuff. Tangible stuff. You might think about the 
spacecraft, over 800 kilograms (nearly one ton) of structure and electronics 
gear. You might think of giant antennas to receive the signals from outer 
space. You might think of control centers with video displays and red, green, 
and yellow lights. 

But you might not think of people, plans, and coordination. You should. 
This is the intangible stuff from which planetary missions are made. The 
theme of Voyager's people runs through this chapter of the Guide. Their 
performance through August 1989 will determine if Voyager Neptune 
earns a gold medal. 

The gold medal, reminiscent of the XXIVth Olympiad held in Seoul in 
1988, provides us a good comparison. The Voyager encounter with Neptune 
is an Olympian event, in terms of cost, complexity, number of people, and 
world-wide involvement. 


Start with the Voyager scientists. There are 1 30 Voyager investigators 
at universities, observatories, and aerospace companies scattered about the 
United States, Canada, England, France, West Germany, Italy, and the 
Soviet Union. The investigators are Voyager's competitors, by an elimina- 
tion process at least as intense as the Olympic trials. They are supported by 
half again their number of research assistants and students. The investiga- 
tors formulate the basic questions to be answered about Neptune, its 
satellites, and its neighborhood. 

These questions require that Voyager 2 make specific observations at 
Neptune under just the right conditions. To match questions with observa- 
tions requires a vast amount of information about the spacecraft and about 
Neptune. For example, just where is the spacecraft, and in what direction 
must the spacecraft look to see the various objects of interest? Determining 
the spacecraft position is the job of a 10-member group of spacecraft 
navigators at JPL who determine both the spacecraft location and the 
thruster firings needed to correct the location. 



Figure 3 - 1 . Charting Voyager's path through the solar system is a precise science , 
with only the occasional need to make artful choices among candidate “solutions . " 

Determining the locations of Triton and Nereid is initially the job of 
another 10-member group of orbit experts, some at JPL and some at places 
like the Universities of Texas and Virginia. They take the latest information 
available to refine the ephemeris (positions) of Neptune and its satellites. 
Remember, Neptune travels around the Sun so slowly that less than a 
Neptunian year has passed since its discovery, and less than three Neptu- 
nian months have passed since the discovery of its satellite Nereid by G. P. 
Kuiper in 1949. IPs no wonder that there is still some uncertainty about 
precise locations, but these ephemeris experts can typically predict satel- 
lites positions to accuracies of 3000 km (1900 mi) or better many months in 
advance of the encounter. The predicted position of Nereid, however, has a 
much greater uncertainty. 

TIME: -0 Days, 2:55:00 
1989/08/25 1:05 GMT (SCT) 

The spacecraft navigators are after even better 
accuracies. They are equipped with some pretty fancy 
orbit determination computer programs that solve for 
the simultaneous positions of all bodies. Their most 
important data for updating the orbital elements of 
the satellites will be Voyager-2 images of the satellites 
against a known star background. By using the 




211698 KM 

narrow-angle TV camera to shoot satellite-star images until a few days 
before closest approach, these real-time navigators can estimate Triton’s 
position relative to Neptune to 100 km (62 mi) or better. This is equivalent 
to locating the finish line for the marathon to an accuracy of 1/40 of an inch. 

Okay, so we know where everything is. What’s our master plan? We 
have limited resources and a rigid time schedule, but we want the greatest 
possible mission return. Voyager’s Mission Planning Office (MPO) has the 
task of preparing a large collection of “guidelines and constraints” that 
govern how the Project resources and spacecraft consumables will be used to 
achieve high-value science return, while maintaining an acceptable level of 
risk. As suggested by Figure 3-2, the guidelines establish the envelope 
within which the mission sequences will be designed, implemented, and 
executed. The MPO function is analogous to that of the Olympic Organizing 
Committee. The time span is even comparable. Several of the basic decisions 
which were essential to the Neptune encounter were even made in the mid- 
1970s, well before launch. 

Given knowledge of the satellite and target locations with time, scan 
platform pointing and observing conditions can be computed. The Science 
Investigation Support (SIS) Team at JPL, about 20 strong, acts as the focus 

Figure 3-2. Mission planning establishes guidelines for the use of project consumables 
and helps define the envelope within which the sequences will be developed. 


for the detailed science planning. Representatives for each experiment 
interact with investigators and other parts of the Voyager Project. The result 
is an integrated plan for all observations intended. 

It is not uncommon for the investigators to desire conflicting observa- 
tions. To resolve these conflicts, the Voyager Science Steering Group (SSG), 
composed of the Principal Investigators (the leaders of each of the eleven 
Voyager scientific teams), meets regularly at JPL to review the observation 
plans. The members of the SSG make the science decisions required to 
ensure that the observation plans will yield the best Neptune science. 


As soon as the final observation plan is ready, primary responsibility 
passes to the sequence de velopment engineers. The Sequence Team consists 
of about 30 members at JPL who flesh out the observation plans with the 
instrument and spacecraft commands required to produce the desired 
results. The Sequence Team ensures that no operating constraints are 
violated and that all of the instructions to the spacecraft will fit into its 
computer memory. The Voyager-2 sequence computer, the Computer 
Command Subsystem (CCS), has roughly 2500 words of memory reserved for 
sequencing. Two words are required for a simple instruction: one to specify 
the event to take place, the other to specify the time of occurrence. For a 
period of high activity, such as Neptune near-encounter, CCS words are 
always at a premium. 

The Spacecraft Team, about 70 engineers at JPL, is responsible for the 
health and optimal use of the spacecraft. It must analyze the spacecraft 
engineering telemetry to determine how the spacecraft is performing, as well 
as plan the engineering sequences that are needed for the observation plans 
to succeed. Roughly half the Spacecraft Team will be engaged in planning 
at any given time, while the rest will be involved with data analysis. 

Engineering sequences, such as spacecraft maneuvers, are passed from 
the Spacecraft Team planners to the Sequence Team for incorporation into 
the CCS loads. 

The total sequence design process from MPO guidelines through on- 

TIME: -0 Days, 2:50:00 
1989/08/25 1:10 GMT (SCT) 

the-shelf CCS loads typically requires many months 
of technical interactions, give and take, teamwork, 
and decisions. Figure 3-3 is an early cartoon sketch of 
the process, but it is still close enough to today’s per- 
ceptions to warrant inclusion in the Guide. 

Once a CCS load has been built and verified, it 
is ready to be sent to the Voyager-2 spacecraft. The 



206195 KM 


Figure 3-3. People often have humorous ways to view their working 
interrelationships , and Voyager is no exception (early version of “ye olde sequence 
design process”). 

final step by the Sequence Team is to process a CCS load from a text listing 
of commands to the stream of Is and Os (bits) which will actually be received 
by the spacecraft. Stored on magnetic computer tape, this binary command 
stream is passed to the Flight Operations Office (FOO). 

Flight Operations 

The FOO, comprising about 45 engineers located at JPL, is the real 
operator of the spacecraft. The FOO controls all transmissions to the 
spacecraft and receives all data at JPL from the spacecraft. It is responsible 
for coordinating the Voyager Project’s activities with operational organiza- 
tions outside the Voyager Project, such as NASA’s Deep Space Network 
(DSN), JPL’s Multimission Control and Computing Center (MCCC), and 
Goddard Space Flight Center’s NASCOM communications network. Sched- 
uling is one of FOO’s most important activities. If a CCS load contains a 
critical observation of the Neptune atmosphere near encounter, the appro- 
priate DSN ground antennas must be tracking Voyager 2 at that time, or the 
information returned from the spacecraft will fall on deaf ears. 

During the Neptune encounter, the DSN will be tracking not only 
Voyagers 1 and 2, but also Pioneers 10, 11, and 12, and the Magellan probe 
to Venus. It may also be asked to track three earlier Pioneers (6,7, and 8) and 


the International Comet Explorer (ICE). With such a demand for its 
services, the DSN scheduling process is involved, indeed. 

Representatives of the FOO meet with representatives of other projects 
far in advance of the requested coverage dates to hammer out an equitable 
allocation of DSN antenna support to all projects. Using MPO guidelines, 
the tracking schedule that results is used in the generation of observation 
plans by the Science Investigation Support Teams, the Navigation Team, the 
Spacecraft Team, and the Sequence Team. The amount of tracking that a 
spacecraft receives in an}^ period depends on the relative importance of that 
period to its mission. Voyager 2 will receive the full resources of the DSN 
during its Neptune flyby, but will receive only sporadic tracking several 
weeks later as other spacecraft cry for increased tracking support. 


With the command tape prepared and the DSN and NASCOM sched- 
uled, the CCS load can be transmitted, or uplinked, to Voyager 2. The tape 
will be played into the JPL-based Voyager Command System, where the 
command stream will be formatted to the Ground Communication Facility 
(GCF) standards, and sent via GCF to the appropriate DSN Deep Space 
Communications Complex (DSCC) for transmission to the spacecraft. GCF 
uses a combination of communication satellites and conventional surface 
and undersea circuits to link together JPL and the DSCCs, just as the TV 
networks did to broadcast the Seoul Games around the world. As the GCF 
message containing the command stream reaches the DSCC, it is checked for 
correct reception and the GCF formatting bits are removed. It is then routed 
to the transmitting station and sent to the spacecraft. 

There are three DSCCs, located in California, Australia, and Spain. 
These locations were chosen at widely separated longitudes to provide 
essentially continuous tracking capability to any interplanetary spacecraft 
as the Earth rotates. The equipment at each site is similar. 

The Goldstone DSCC is located near Goldstone Dry Lake in the heart 
of the Mojave Desert in California. Three antennas at Goldstone support the 
Voyagers: DSS 12, a 34-m (112-ft) diameter antenna which can both 

1989/08/25 1:15 GMT (SCT) 

TIME: -0 Days, 2:45:00 

transmit and receive; DSS 14, a 70-m (230-ft) diameter 
antenna which can both transmit and receive; and 
DSS 15, a 34-m (112-ft) diameter antenna which can 
only receive. As shown in Figure 3-4, a 70-m antenna 
is quite a large structure. For increased performance, 
more than one antenna can be used simultaneously in 
array to increase the strength of the received signa! 



200684 KM 

from Voyager 2 (see Chapter 9, Engineering Wizardry). The Goldstone 
DSCC is operated for NASA by JPL with a staff of 280 engineers and 
technicians. An additional cadre of managers, development engineers, and 
programmers for the DSN reside at JPL in Pasadena, California. 

The Canberra DSCC is located in the semi-arid rolling hills of New 
South Wales at Tidbinbilla, not far from the Australian Capital Territory of 
Canberra. Three antennas at Canberra support Voyager 2: DSS 42, a 34- 
m (112-ft) transmit/receive station; DSS 43, a 70-m (230-ft) transmit/receive 
station; and DSS 45, a 34-m (112-ft) receive-only station. The Canberra 
DSCC is operated for NASA by 150 engineers and technicians from the 
Australian Department of Science. 

The Madrid DSCC is located in the foothills at Robledo, Spain, near the 
capital city of Madrid. Three antennas at Madrid support Voyager 2: DSS 
61 , a 34-m (1 1 2-ft) transmit/receive antenna; DSS 65, a 34-m (1 1 2-ft) receive- 
only station; and DSS 63, a 70-m (230-ft) transmit/receive antenna. The 
Madrid DSCC is operated for NASA by the Spanish National Institute for 
Aerospace Techniques (INTA) with about 200 engineers and technicians (see 
Figure 3-5). Voice communications, basically a continuous phone call, are 
maintained between all three DSCCs and the Network Operations Control 
center at JPL. 

One would surely think that we’ve got all the receiving antennas we 
could possible need. Not so, however. The Voyager signals are so feeble by 
the time they have crossed 4.5 billion km of space that we can make 
important use of even more giant ears to catch the Voyager news. Helping 
out Goldstone will be the twenty-seven 25-m (82-ft) antennas of the Very 
Large Array (VLA) (shown in Figure 9-1) near Socorro, New Mexico, 
operated by Associated Universities, Inc. as a part of the National Science 
Foundation’s National Radio Astronomy Observatory (NRAO). Because the 
Neptune and Triton closest approach periods occur over the Australian lon- 
gitude, helping out Canberra will be the 64-m Parkes Radio Observatory, 
located some 320 km (200 mi) northwest of the DSN complex. Parkes is 
operated by the Australian Commonwealth Scientific and Industrial Re- 
search Organization (CSIRO). Finally, the 64-m radio observatory antenna 
at Usuda, Japan, will provide additional tracking during the critical radio 
science observations near closest approach. The Japanese Institute of 
Astronautical Science (ISAS) operates the Usuda antenna on the island on 
Honshu in the hills west of Tokyo. 

We left the command load just radiating from the DSS antenna on its 
way to Voyager 2. Even at the speed of light, the first command will not 
arrive at Voyager for 4.1 hours, and acknowledgement of its receipt can’t be 



Figure 3-4. If we could move the 70-m DSN tracking antenna from its Goldstone, 
California , desert location to the football field inside the Pasadena Rose Bowl, this 
is how large it would appear! Big ears are needed at Earth Base to hear the feeble 
signals from a remote spacecraft. 

seen at Earth until 8.2 hours after it was sent. Data telemetered from 
Voyager 2 are 4.1 hours old the instant they are received. This time lag 
complicates operations greatly. Imagine driving a car where the gauge 
readings and even the sights seen out of the windows are over four hours old, 
and the response to turning the steering wheel or applying the brake is four 
hours in the future! Luckily, there is less traffic on the way to Neptune than 
there is on the Los Angeles freeways. 

Receiving Data 

The data received are of enormous value. First there is telemetry, 
which consists of information describing the performance of the spacecraft 
and its instruments and the science measurements 
themselves. The telemetry is identified with space- 
craft time so that a complete reconstruction of the 
state of the spacecraft can be used to determine the 
health of all engineering and science subsystems, and 
to aid in science interpretation. 

TIME: -0 Days, 2:40:00 
1989/08/25 1:20 GMT (SCT) 


195165 KM 


Figure 3-5. The Voyager family consists of full-time Voyager people (O), part-time 
Voyager people (D) and multimission support people ( A ). The tracking complex 
staffing levels shown represent the full complement of personnel who support not 
only Voyager ; but several other missions as well. 

Second there are navigation data. One type are doppler data, which are 
contained in the Voyager-2 radio signal itself and are dependent upon the 
relative motion between the spacecraft and tracking antenna. Another is 
range data, which provides a distance measurement from the spacecraft to 
the tracking antenna. A third uses simultaneous tracking by two stations 
of first the spacecraft and then a quasar of known characteristics to get a 
different type of doppler data. Finally, optical navigation video images relate 
the spacecraft and planetary body positions to their directions relative to 
known stars. 

All of these data are both relayed (via GCF) to JPL and recorded at the 
DSCC. This allows any gaps to be recovered after the fact if any data are lost 
on the way to their ultimate user. The first users are the Mission Control 
Team, the Navigation Team, and the Spacecraft Team. Essential engineer- 
ing measurements and status indicators are displayed as they are received 
so that corrective action may be initiated at any sign of a problem. 

The entire data stream is routed through the Ground Data System 
(GDS), consisting of the DSN, NASCOM, MCCC and a number of Voyager 


Project data systems. The data are finally processed by an assemblage of 
some 55 people and various computers at JPL. Here, the telemetry is read 
and identified, and reassembled by measurement rather than as a serial 
stream. If coding has been applied, it is decoded. Imaging is transferred to 
the Multimission Image Processing Subsystem (MIPS) to be converted from 
digital picture elements into pictures. If the imaging has been compressed 
(see Chapter 9), MIPS reverses the compression. During this process the 
images can be enhanced to bring out subtle features and, in some cases, even 
corrected for errors. 

All imaging and non-imaging data are collected and processed into 
Experiment Data Records (EDRs), which contain all available science and 
engineering data from a given instrument. The EDRs are the basic delivery 
of observed data to the investigators. A companion record, the Supplemen- 
tary Experiment Data Record (SEDR), contains the best estimate of the 
conditions under which the observations were taken. 

If you would like to learn more about how we route the steady stream 
of data bits arriving at Earth from the Voyager transmissions, please refer 
to Figures 6-6 and 6-7, plus the accompanying text in Chapter 6. 

The Results 

As the encounter with Neptune approaches, delivery of the data to the 
investigators will become easier because the scientists will be migrating to 
JPL. As the pace quickens, monthly investigator meetings will become daily 
meetings, and ideas will be exchanged furiously as they strive to understand 
the details of a new planetary system. When the last of the Neptune data are 
safely acquired, the scientists will retreat with the data to their own 
institutions to begin the intensive process of converting measurements into 
answers to those fundamental questions raised in Chapter 2. 

That won’t be the end, however. Archived, the Voyager-2 Neptune data 
will be available to scientists the world over through the National Space 
Science Data Center at the NASA/Goddard Space Flight Center in Green- 
belt, Maryland. It is a safe bet that somewhere today there are tens or 
hundreds of elementary school students who, in the early twenty-first 
century, will be writing doctoral dissertations based 
on their study of the Voyager-2 Neptune data. And 
that, in the year of the XXVIIth Olympiad, will be 
Voyager’s real gold medal. 

TIME: -0 Days. 235:00 
1989/08/25 1:25 GMT (SCT) 


189638 KM 


One wonders if their messages came long ago, hurtling into 
the swamp of the steaming coal forests, the bright projectile 
clambered over by hissing reptiles, and the delicate 
instruments running mindlessly down with no report. 

Loren Eiseley 


The mission objectives can be met only by delivering the spacecraft to 
the Neptunian system along the chosen flight path, properly orienting the 
spacecraft and pointing its instruments at the desired celestial bodies, 
powering the instruments, giving instructions to them, and channeling the 
science information gathered to the radio subsystem for transmission to 
Earth. In other words, a pretty complex machine is necessary to support the 
science instruments. Several years before launch, a spacecraft design team 
(Figure 4-1) worked out the basic requirements for this amazing machine 
and, judging by its success to date, they did a first-class job. 

Figure 4-1. Before launch, a spacecraft design team did a lot of brainstorming to 
hammer out the dozens of major considerations (and thousands of smaller details) 
needed to design and build the amazing Voyager robots. 



The basic structure of the spacecraft is called the “bus,'' which carries 
the various engineering subsystems and scientific instruments. It is like a 
large ten-sided box, which can be seen in Figure 4-2. The centerline of the 
bus is called the z-axis, or roll axis. The spacecraft will usually be aligned 
so this z-axis (and thus the High Gain Antenna) points to Earth. The 
spacecraft is designed to roll about this axis by firing small thrusters which 
are attached to the bus. The thrusters are fueled by a liquid called hydrazine. 

Each of the ten sides of the bus contains a compartment (a bay) that 
houses various electronic assemblies. Bay 1 , for example, contains the radio 
transmitters. The bays are numbered from 1 to 10 (numbered clockwise as 
seen from Earth). 

Two additional turn axes, at right angles to the roll axis and to each 
other, are needed to give the spacecraft full maneuverability. These are the 
x-axis (pitch) and the y-axis (yaw). The booms supporting the nuclear power 
sources and the scan platform lie along the y-axis. 

High Gain Antenna (HGA) 

On many spacecraft, a small antenna dish sits on the spacecraft bus 
and is steerable. But Voyager is different; it may almost be said that the 
spacecraft bus sits on the High Gain Antenna (see Figure 4-2.) 

The HGA transmits data to Earth on two frequency channels (the 
downlink). One, at about 8.4 gigahertz (8,400,000,000 cycles/sec), is the X- 
band channel and contains science and engineering data. For comparison, 
the FM radio band is centered around 100 megahertz (100,000,000 cycles/ 
sec). The X-band downlink science data rates vary from 4.8 to 21 .6 Kbps (ki- 
lobits per second). The other channel, around 2.3 gigahertz, is in the S-band, 
and contains only engineering data on the health and state of the spacecraft 
at the low rate of 40 bps. 

The HGA is so called because signal strength is gained by focusing the 
radio energy into a highly concentrated narrow beam. The half-power points 
of the HGA are 0.5 degrees off axis for the X-band and 2.3 degrees for the S- 
band (i.e., if the antenna strays as much as 0.5 degrees off point, the X-band 
signal strength drops by half). There is also a Low 
Gain Antenna, but it is not used anymore except in 
response to certain faults involving loss of spacecraft 

TIME: -0 Days, 2:30:00 
1989/08/25 1:30 GMT (SCT) 


Figure 4-2. The Voyager spacecraft has a launch mass of 825 kg, is nuclear-electric 
powered, consists of about five million equivalent electronic parts, and uses 
onboard computer fault detection and response to protect itself. 


Spacecraft Attitude Control 

Relative to attitude control, the two most common types of spacecraft 
are spin-stabilized and three-axis-stabilized. The former, such as the Pio- 
neer spacecraft, obtain stabilization by spinning so that the entire spacecraft 
acts as a steady gyroscope. Three-axis-stabilized spacecraft, such as Voy- 
ager, maintain a fixed orientation, or attitude, in space except when maneu- 

Spacecraft stabilization, as well as spacecraft maneuvering, is con- 
trolled by an onboard computer called the Attitude and Articulation Control 
Subsystem (AACS). This computer also controls scan platform motion. 

Voyager has two ways of maintaining its attitude: by gyro control and 
by celestial control. Gyro control is used for special purposes and short 
periods of time, up to several hours. 

In the celestial control mode, Voyager maintains its fixed attitude in 
space by viewing the Sun and a bright star such as Canopus, Alkaid, or 
Achernar. If the spacecraft should drift from its proper orientation by more 
than a certain angle (called the deadband), the AACS will issue commands 
to fire the tiny attitude-control thrusters to bring it back to proper orienta- 
tion (see Figure 4-3). Canopus is used almost exclusively as the reference 
star by Voyager 2 during interplanetary cruise (Voyager 1 uses Rigel 
Kentaurus). Canopus, the second brightest star in the sky, is a southern 
hemisphere star and is barely visible from the southern United States. 

The sensor instruments used to track the Sun and star are the Sun 
Sensor (mounted on the HGA) and the Canopus Star Tracker (CST), so 
named because Canopus is the preferred star to use whenever possible. 
Figure 4-3 shows the celestial sensor and gyro accuracies, the limit cycle 
deadband, and the final scan platform pointing accuracy. 

Spacecraft Maneuvers 

There are many types of spacecraft maneuvers. We choose one that is 
fairly simple, and also somewhat common, to use as an example— this is the 
Stellar Reference Change. 

There are times during encounter when Canopus is not suitable as a 

1989/08/25 1:35 GMT (SCT) 

TIME: -0 Days. 225:00 

reference star for Voyager 2. For example, the planet 
might be on the other side of the bus from the scan 
platform when Canopus is the lock star. Imaging the 
planet would then be impossible because spacecraft 
parts block the view. In this case, an alternate star 



178556 KM 

★ Star 

Figure 4-3. When it comes to pointing precision , the Voyager spacecraft is quite a 
remarkable machine. 

(which is on the other side of the sky from Canopus) is chosen, and the Stellar 
Reference Change maneuver is required. 

The maneuver is controlled by the AACS computer. First, the space- 
craft goes to gyro control (all-axes inertial mode); then the AACS fires the hy- 
drazine thrusters to start the spacecraft turn about the roll axis. The turn 
rate is precisely chosen to be either the nominal turn rate (0.18 degrees per 
second) or a new higher turn rate (0.30 degrees per second). 

After the spacecraft has turned through the prescribed angle, the 
AACS fires the opposite set of thrusters to halt the turn. Since the roll turn 
is about the axis pointing toward Earth, the Sun will have “coned” around 
this axis and reached a different spot on the spacecraft’s Sun Sensor plate. 
The Sun Sensor then locks onto the Sun in its new position. (This displace- 
ment of the Sun from the roll axis is called the Sun Sensor bias.) Finally, the 
star tracker locks onto the new reference star, and the spacecraft is returned 
from gyro control to celestial control. 

Scan Platform Pointing 

Several appendages are attached to the spacecraft bus. These are the 
HGA (discussed above), the magnetometer boom, the PRA antennae (rabbit 


ears), the RTG boom (supplying power), and the scan platform. These are 

shown in Figure 4-2. 

Four of the science instruments are on the scan platform: namely those 
that need to be pointed at a target body (the planet, a star, rings, or one of 
the satellites). A glance at Figure 4-2 will show you why it is necessary to 
mount these instruments on such a long boom. If they were mounted on the 
bus, they could not look backwards (during post-encounter) because the 
HGA would block their view. The spacecraft mass distribution is balanced 
by placing the scan platform on the other side of the bus from the radioactive 
power source. 

The scan platform has motors and gears (called actuators) which slew 
the platform to point in various directions. If you imagine “up” as being in 
the direction of the HGA boresight (generally toward Earth), then a motion 
up or down is accomplished by an elevation slew, and a motion to the right 
or left is accomplished by an azimuth slew. These are called, for short, El 
slews and A z slews. The locations and components of the actuators are 
shown in Figures 4-4 and 4-5. 

About 102 minutes after Voyager 2’s closest approach to Saturn in 
1981, the azimuth motion of the scan platform unexpectedly halted, and 
science data were lost from the instruments that require pointing. Appar- 
ently this seizure was due to heavy use of high-rate slews to move the scan 
platform at 1 deg/sec. A vital lubricant probably migrated away from a tiny 
shaft-gear interface (spinning at 170 rpm), resulting in galling (wearing 
away from friction) and debris buildup, and finally leading to the seizure. 
Scan platform motion was resumed in two days, but analysis and testing 
leading to a failure model, to a strategy to monitor the actuator’s health, and 
to guidelines for safe use of the actuator were completed over three years. 

Needless to say, the faster slews will not be used during the Neptune 
encounter, except for eight medium-rate (0.33 deg/sec) slews used to capture 
critical science observations. All other slews will not exceed the low rate of 
0.08 deg/sec. Nevertheless, the scan platform motion will be monitored quite 
closely by the Torque Margin Test (see Chapters 8 and 9). A contingency 
near-encounter sequence (R951) has been prepared for use just in case an 
azimuth actuator problem is experienced. 

TIME: -O Days, 2:20:00 
1989/08/25 1:40 GMT (SCT) 

TO NEPTUNE: 173001 KM 

Power Subsystem 

Spacecraft electrical power is supplied from three 
Radioisotope Thermoelectric Generators (RTGs), 
which are miniature nuclear power plants that con- 
vert about 7000 watts of heat into some 400 watts of 


Azimuth Axis 

Azimuth Axis Actuator 

Scan Platform Structure 

Imaging Science Subsystem 
Wide Angle (ISS-WA) 

Elevation Axis x 

Imaging Science Subsystem 
Narrow Angle (ISS-NA) 

Photopolarimeter (PPS) 

Infrared Interferometer 
Spectrometer and 
Radiometer (IRIS) 

Elevation Axis 

Spectrometer (UVS) 

Figure 4-4. This view of the Voyager scan platform shows the locations of the 
two electric motors and gear trains , known as “ actuators that drive the 
platform to look in different directions. 

Fine Pots (4) 

Output .. \ 


^ Clutch 



Coarse Pot (2) 

Fine Pot Telemetry (4) 
Stepper Motor 




Cs i 

v r » s( 


Output Shaft x 

Coarse Pot 
Telemetry (2) 

Stepper Motor 

Figure 4-5 . Small electric motors drive the Voyager scan platform about “azimuth” 
and “elevation ” axes. Voyager 2’s azimuth actuator stuck shortly after the Saturn 
encounter, but was used for the Uranus encounter and will be used for the Neptune 




electricity. These lie along the RTG boom, away from the spacecraft bus and 
opposite the scan platform (see Figure 4-2.) 

At launch the power output from the RTGs was 475 watts. However, the 
power output decreases by about 7 watts each year due to several causes, in- 
cluding the half-life of the fissionable plutonium dioxide and degradation of 
the silicon-germanium thermocouples. By the time Voyager 2 reaches 
Neptune, the RTG power output will be down to about 370 watts. 

The spacecraft power load is constrained to be less than the RTG 
output, and excess power is dissipated through the shunt radiator as heat. 
The difference between the available power and the power used in running 
the spacecraft is called the “power margin.” Since the power available is 
substantially less for Neptune than for previous encounters, great care is 
taken to plan the power management strategy. For example, the S-band 
high-power state is no longer used. 

Project guidelines require that this power margin be kept fairly large 
(above 12 watts) as a safeguard against power surges or miscalculations 
which might cause the spacecraft to try to draw more power than is available. 
But, were this to happen, the onboard computer has a fault protection 
algorithm (FPA) that could turn off power to some subsystems to reduce 
power consumption. This would be a major inconvenience and, if it hap- 
pened during encounter, would cause loss of science data. 

Data Storage Subsystem 

There are occasions when the Voyager spacecraft cannot immediately 
send science telemetry data to Earth. These occasions could occur during a 
spacecraft maneuver when the HGA is not pointed at Earth, or during the 
time the spacecraft is behind the planet as seen from Earth (Earth occulta- 
tion). Also, it is no longer possible to send certain types of data (such as PWS 
and PRA high-rate frames) directly into the telemetry stream because the 
data rate is too high to be received without error. In all these instances, the 
Digital Tape Recorder (DTR) is available to store the data for later playback 
to Earth. 

The DTR has three speeds in use at Neptune encounter. But, rather 
than citing the speed in inches per second, as for 
conventional tape recorders, the speed is cited in 
units of information per second, kilobits per second 
(Kbps). The three speeds are 115.2 Kbps (record 
only), 21.6 Kbps (playback only), and 7.2 Kbps (both 
record and playback). There are eight tracks on the 
DTR. Each of these can hold up to 12 images if only 


TIME: -O Days. 2:15:00 
1989/08/25 1:45 GMT (SCT) 


167437 KM 

images are recorded. This is seldom the case, since data from other 
instruments need to be recorded also. 

As you can imagine, the experimenters often like to record more data 
than the tape recorder has the capability to store. Thus, DTR data 
management is a critical concern. It is important to play the data back 
quickly so that the tape recorder can be filled again. But, playbacks interfere 
with science gathering and require certain DSN configurations that are not 
always available. So data management during busy periods remains a 
challenging task. 

Spacecraft Receiver 

Periodically, instructions are sent (uplinked) from the ground to the 
spacecraft. These instructions, called commands, are modulated (superim- 
posed) onto the radio signal and are transmitted at 16 bits per second by one 
of the DSN tracking stations. Traveling at the speed of light, they will reach 
the spacecraft at Neptune in just over four hours. The radio signal carrying 
the commands is received by the spacecraft HGA and sent to the receiver. 
The receiver extracts the command subcarrier from the carrier signal and 
sends it to the Command Detector Unit (CDU). Here the commands are 
demodulated (removed from the subcarrier) and converted to digital form. 
The commands are then sent to the Computer Command Subsystem (CCS). 

On April 6, 1978, a failure protection algorithm in Voyager 2’s CCS 
automatically switched from the prime to the backup receiver. But a 
tracking loop capacitor had failed in the backup receiver. Soon after a com- 
manded return to the prime receiver, the prime receiver suddenly failed. 
Seven days later, the fault protection algorithm switched back to the crippled 
backup receiver. Because of these two failures, more complicated proce- 
dures are used for commanding Voyager 2 than are used for Voyager 1. 

The receivers were designed to lock onto the signal in order to follow 
shifts in frequency, but this function is no longer possible on Voyager 2. 
(These shifts in frequency are Doppler shifts that result from changes in the 
relative velocity between the spacecraft and the DSN antenna, due primar- 
ily to the Earth’s rotation.) In commanding Voyager 1 , for example, the DSN 
transmits at a constant frequency and the receiver locks onto and follows the 
moving frequency. 

However, for Voyager 2, the failed tracking loop capacitor makes it 
necessary for the received signal to be at a constant frequency. To accommo- 
date the Doppler shifts, it is then necessary for the DSN tracking station to 
transmit a moving frequency. If the transmitted frequency is not within 96 
Hertz (cycles/sec) of the receiver rest frequency, then Voyager 2 will turn a 


deaf ear on instructions from Earth. Furthermore, an unpredicted tempera- 
ture change of as little as 0.25° C (0.45° F) in the receiver will shift the rest 
frequency by 96 Hertz from its predicted value. Temperature changes in the 
receiver can be caused by a change in the spacecraft’s power status or by the 
Sun’s heat on the receiver bay. The Spacecraft Team may schedule a 
“command moratorium” period up to 48 hours before or after critical events 
to assure that the receiver temperature has had a chance to stabilize. All of 
these factors complicate the process of sending commands to Voyager 2. 

If the second receiver were to fail, there would be no way to command 
the spacecraft to execute further activity. We would not be able to point the 
scan platform instruments at their targets nor point the HGA at Earth. We 
could not have a successful encounter. To protect against failure of the 
remaining receiver, a special CCS Load has been placed in the CCS. This 
contingency Back-up Mission Load (BML) contains a few commands that 
will allow some science to be gathered in the event that the regular encounter 
CCS Loads cannot be received by the spacecraft because of receiver failure. 
There are several designs for BMLs, each resident in the CCS over a different 
interval of time. 

As serious as all this sounds, remember that despite these problems, 
Voyager 2 has so far completed successful encounters with Jupiter, Saturn, 
and Uranus, and has begun its task of revealing the mystic Neptune. 

Computer Command Subsystem (CCS) 

The CCS consists of two identical computer processors, their software 
algorithms, and some associated electronic hardware. The CCS is the 
central controller of the spacecraft (the brain of the spacecraft, if you will). 
Figure 4-6 shows the CCS in relation to the AACS and Flight Data Subsys- 
tem (FDS) computers, as well as to the DTR (the main component in the Data 
Storage Subsystem) and other subsystems. 

The CCS has two main functions: to carry out instructions from the 
ground to operate the spacecraft and gather science data; and to be ever alert 
for and to respond to any problem with any of the spacecraft subsystems. 

The latter of these CCS functions is carried out by a series of software 

TIME: -0 Days. 2:10:00 
1989/08/25 1:50 GMT (SCT) 

routines called Fault Protection Algorithms (FPAs). 
These algorithms, which occupy roughly twenty per- 
cent of the CCS memory, make the spacecraft semi- 
autonomous and able to act quickly to protect itself. 
This is important because of the long delay time 



161863 KM 

required for a response to the problem from Earth: over eight hours round- 
trip light time (at Neptune closest approach), plus the reaction time of the 
engineers to detect the problem and prepare the proper response. In many 
instances such a delay would be intolerable. 

The other CCS function, storing and processing commands from Earth, 
allows the spacecraft to act as an intelligent robot to carry out its science- 
gathering functions in strict accordance with the carefully developed mis- 
sion plan. 

It is convenient to send up one transmission to the spacecraft which 
contains most or all of the commands needed to operate it for periods of time 
ranging from 30 days during the Observatory Phase to only two days during 
Near Encounter. Each of these transmissions nearly fills the remaining 
(non-FPA) memory in the two CCS computers. The contents of each of these 
transmissions is called a CCS Load and is given an identifying number. “A” 
loads refer to Voyager 1 , and “B” loads to Voyager 2; for example, A821 , B901 , 
B902, etc. Chapter 6 contains a timeline which shows each of the encounter 
CCS Loads. 

• Attitude 

• Sensors 

• Gyros 

• Scan Act. 

• Thrusters 

Computer Command 
Subsys (CCS) 





1 Instr2 

Instr 1 


n Telemetry 

Figure 4-6. Voyager's three computer subsystems contain nearly 33,000 words of 
memory storage, with the Computer Command Subsystem (CCS) directing most of 
the activities. 


Flight Data Subsystem (FDS) 

The onboard FDS, comprising two reprogrammable digital computers 
and associated encoding hardware, collects and formats the spacecraft’s sci- 
ence and engineering telemetry data for transmission to Earth. 

The engineering data, generally at 40 bits per second, are on the S-band 
downlink and are embedded in science telemetry data on the X-band 
downlink as well. Engineering data provide the status of the instruments, 
health of the various spacecraft subsystems, and spacecraft attitude and 
scan platform position. 

The science data (the results of the science observations) are on the 
high-rate data channel (4.8 to 21.6 Kbps) and are downlinked only on X- 

The FDS “encodes” the telemetry data by adding redundant bits to the 
telemetry data in such a way that bits lost in the static may be reconstructed 
(i.e., intelligently guessed at). For example, the Golay encoding process adds 
3600 bits to every 3600 bits of raw science data telemetered to Earth. 
However, the more efficient Reed-Solomon encoding process only adds 1200 
bits to every 3600 bits of raw science data sent to Earth. (See Chapter 9 for 
further discussion of encoding.) 

The FDS also does some special data processing on the picture data in 
a process called image data compression (IDC), described in Chapter 9. 
Rather than use the secondary FDS processor as a “hot backup” for the 
primary computer (both containing identical software), the FDS computers 
are used in a “dual processor mode” in which the primary unit samples and 
formats the general science data, while the secondary unit compresses the 
imaging data. Here, the two processors work in parallel and each performs 
different functions, effectively doubling the computer memory available for 
computing tasks. 

The FDS also provides appropriate instrument control for the science 
instruments and the digital tape recorder. For example, control data for the 
imaging instruments in an “imaging parameter table” in FDS memory 
provides instructions for imaging shutter modes, filter choices, and exposure 
levels for each camera. 

TIME: -0 Days, 2:05:00 
1989/08/25 1:55 GMT (SCT) 

One of Voyager 2’s FDS computer memories has 
lost a block of 256 memory locations, out of a total of 
8192. This is a rather minor failure. However, the 
loss of 512 more words from the primary memory 
would be serious, for it would mean we would have to 
abandon the dual processor mode, and hence, the 
valuable IDC capability. 



156279 KM 

Science Instruments 

There are eleven scientific investigations on Voyager 2, and the loca- 
tions of their instruments are shown in Figure 4-2. Figure 5-5 gives sketches 
and brief descriptions of each of the instruments. Of these, only four are not 
located on the scan platform or its supporting boom. The Magnetometers 
use their own boom; the Planetary Radio Astronomy (PRA) experiment 
shares the rabbit ear antennas with the Plasma Wave Subsystem (PWS); 
and the Radio Science Subsystem (RSS) uses the radio beams from the HGA. 

The four instruments on the scan platform require accurate pointing; 
these are the Imaging Science Subsystem (ISS) wide- and narrow-angle cam- 
eras, the Ultraviolet Spectrometer (UVS), the Infrared Interferometer 
Spectrometer (IRIS), and the Photopolarimeter Subsystem (PPS). Figure 5- 
6 shows the relative fields of view of these instruments, and their pointing 
offsets from each other. These instruments may all make observations 

Note the long, rectangular UVS slit. For some observations this slit 
needs to be aligned in a particular direction, requiring a spacecraft roll 

The remaining three instruments, mounted along the scan platform 
boom, are fields and particles experiments. These are the Cosmic Ray 
Subsystem (CRS), the Low-Energy Charged Particle experiment (LECP), 
and the Plasma Subsystem (PLS). 

All of these experiments (except RSS) send their observational data to 
the FDS to be formatted into telemetry. All of them, except RSS and ISS, con- 
tribute data to a telemetry format called “general science and engineering” 
(GS&E). This GS&E telemetry data mode has a downlink data rate of 4800 
or 7200 bps, depending on the type of encoding the FDS uses, but the 
information content (symbols per second) is 3600 bps. The PRA, for example, 
contributes 266 bps, and the IRIS contributes 1120 bps, out of this total of 
3600 bps. 

The ISS has several data formats of its own at higher data rates because 
of the large number of data bits required to define a picture (5.12 million, if 
uncompressed!). Real-time data rates of 8400 and 14,400 bps allow the 
images to be immediately returned to Earth in the telemetry stream. Non- 
compressed images may be recorded on the tape recorder at 115,200 bps, 
then returned to Earth at a later time at a slower playback rate. Two other 
experiments, PRA and PWS, can also provide a high data rate, and periodi- 
cally short bursts of these data are put onto the DTR at 1 1 5,200 bps for later 
playback at a slower rate. 


As you can see, the Voyager spacecraft is a sophisticated machine. It 
must have a broad spectrum of capabilities to deliver the scientific sensors 
to their desired target geometries, collect the scientific data, and return 
these data to Earth. The scientific sensors are its vital payload and are 
sophisticated devices, as we shall see in Chapter 5. 

TIME: -0 Days, 2:00:00 
1989/08/25 1:60 GMT (SCT) 



150685 KM 

The most beautiful experience we can have is the mysterious. 
It is the fundamental emotion that stands at the cradle of 
true art and true science. 



In Chapter 2, you prepared a shopping list of things you would like to 
find out about the Neptune realm. You know that everything you learn 
comes via one of your four senses. The Voyager spacecraft has 11 sets of 
sensory devices, which can be conveniently divided up into two types: those 
that point at something (called target body sensors) and those that don’t 
(called fields, waves, and particles sensors). There are five sets of target body, 
one fields, two waves, and three particles sensors. 

This chapter contains a description of how each of the sensors works, 
a summary of their engineering characteristics, what types of new knowl- 
edge each of the sensors can provide, and finally, a wrap-up discussion of the 
fundamental physics upon which the sensors are based. 

Imaging Science Subsystem (ISS) 

Humans, like most of Earth’s creatures, have evolved with the ability 
to see a certain kind of light, called visible light. There are many other types 
of “light” which are invisible to our eyes, such as ultraviolet, infrared, and 
radio waves. It is most natural, therefore, when sending a spacecraft off to 
unknown places, for humans to include at least one sensor that is sensitive 
to visible light. Voyager has two sets of science sensors designed primarily 
for visible light operation: the Imaging Science Subsystem (ISS) and the 
Photopolarimeter Subsystem (PPS). 

The ISS consists of two “TV” cameras and associated control electron- 
ics. The ISS functions much like a pair of video cameras. Both the ISS and 
a video camera work on exactly the same principle of nature by recording the 
intensity of light reflected or emitted from the object one is photographing. 
A video camera converts the light into electronic signals which are recorded 
on a video cassette (both of these functions are performed by a camcorder). 
The ISS also converts the light into electrical signals, which are either sent 
directly to the Earth or stored by the spacecraft on the equivalent of a VCR. 

With a normal video camera you have the ability to configure the 
camera to take a wide variety of pictures under a wide variety of circum- 
stances. Available to the photographer are various lenses, filters, aperture 
settings, exposure times, and framing speeds. The ISS has many of the same 


Instead of interchangeable lenses, the ISS has two separate cameras: 
one with a 200mm focal length lens of 60mm aperture and the other with a 
1500mm focal length lens of 176mm aperture. To an Earth-based photogra- 
pher, both would be considered telephoto cameras (normal focal length being 
55mm), but to the Voyager Imaging Team the former is known as the “wide- 
angle” camera and the latter is known as the “narrow-angle” camera. On 
each ISS camera, the lens is fixed. The ISS has the ability to shutter pictures 
one after another or to shutter pictures at widely separated times. 

Each ISS camera has eight different filters. Each camera has a “clear 
filter” that permits the greatest amount of light to pass through to the light- 
sensitive surface. The other filters permit specified types of light to pass 
through and block all other types of light from reaching the camera’s 
detector. Both cameras have violet, blue, orange, and green filters. The 
narrow-angle camera also has an ultraviolet filter and duplicate clear and 
green filters. The wide-angle camera contains three filters explicitly de- 
signed to detect sodium near Io, methane at both Jupiter and Saturn, and 
methane at Uranus and Neptune, respectively. 

Both ISS cameras are fixed aperture devices. However, one may vary 
the exposure time from 0.005 seconds to 61 seconds. Time exposures which 
are longer than normal exposures by integer multiples of 48 seconds are also 
possible. This capability is critical because the sunlight reaching Neptune 
is roughly 36 times dimmer than at Jupiter. The ISS cameras store pictures 
in the form of electrical impulses. The primary differences between the ISS 
and the familiar video cameras are the number of scan lines, the time 
required to scan the image, and the way color is produced. The ISS has 800 
scan lines, while camcorders typically have only 525. The ISS needs from 48 
seconds to several minutes to “read out” the image, while video cameras scan 
an image 60 times each second. 

Concerning color, video cameras do this very easily, while the ISS must 
take black and white pictures through three different color filters. After 
reconstruction back on Earth, the composite photograph can be in “natural” 
color — that is, what the human eye would see. However, it can also be proc- 
essed to greatly enhance color contrast or to represent the scene in “false 
color” to emphasize particular features. 

The ISS is used to observe and record the visible 
characteristics of planets, atmospheres, moons, and 
rings. These visible characteristics include the sizes, 
colors, brightnesses, and surface textures of these 
objects. In addition, groups of ISS pictures are used 
to map the surfaces of moons. 


TO NEPTUNE: 145080 KM 

TIME: -0 Days, 1:55:00 
1989/08/25 2:05 GMT (SCT) 

The Voyager Navigation Team uses ISS pictures of various moons 
against backgrounds of stars with known positions to accurately determine 
where the spacecraft was located at the time the picture was taken. This 
technique is known as optical navigation. 

Infrared Interferometer Spectrometer and Radiometer (IRIS) 

The IRIS is a very specialized type of light meter. It comes equipped 
with a large, permanently attached reflecting telescope. 

The IRIS uses a sensor that can “see” infrared light. Infrared means 
less than or below red. Infrared is light that is next to and below (in 
frequency) the red light that our eyes can see. The IRIS actually acts as three 
separate instruments. First, the IRIS is a very sophisticated thermometer. 
Second, the IRIS is a device that can determine when certain types of 
elements or compounds are present in an atmosphere or on a surface. Third, 
it uses a separate radiometer to measure the total amount of sunlight 
reflected by a body at ultraviolet, visible, and infrared frequencies. 

Any solid, liquid, or gas that has a temperature above absolute zero 
emits heat energy. The amount and “color” (wavelength) of heat energy that 
the substance emits is dependent, among other things, upon its temperature. 
The IRIS can determine the distribution of heat energy a body is emitting, 
which then allows us to determine the temperature of that body or substance. 
In the special case of an atmosphere, the IRIS can determine the tempera- 
ture of the atmosphere at various altitudes, producing what is called a 
temperature profile. 

By measuring the total amount of heat energy that a planet is emitting 
and the total amount of sunlight reflected, and comparing this to the total 
amount of energy received from the Sun, scientists can determine if heat 
from the interior of the planet is escaping. Jupiter, Saturn, and Neptune 
emit about twice as much heat energy as they receive. Mercury, Venus, 
Earth, Mars, and Uranus show little or no evidence of heat generated in their 
interiors and, therefore, reradiate to space very nearly the same amount of 
heat energy they absorb from the Sun. 

The IRIS can also determine if certain elements and molecules are 
present in a particular atmosphere or on a particular surface. The physical 
principle that permits this type of element/molecule determination is the 
following. Remember that atoms consist of one or more protons plus neu- 
trons (only hydrogen has no neutrons) in a nucleus, surrounded by the same 
number of “orbiting” electrons as there are protons. Molecules consist of two 
or more atoms bound together by electrical forces. Light energy (for example, 
from the Sun) may be absorbed by an atom or molecule, which then becomes 


Figure 5-1. The planet absorbs the visible and ultraviolet energy from “sunlight," 
then emits infrared “light” which the IRIS can “see." Certain molecules in the 
cooler, overlying atmosphere absorb some colors of infrared “light.'' 

unstable because it has excess energy. The atom or molecule releases the 
excess by emitting the energy (Figure 5-1 ). If the emitted energy is infrared 
energy, the IRIS can detect the emission. Continuous infrared (heat) energy 
being emitted from deeper, warmer layers of an atmosphere is selectively 
absorbed at discrete infrared colors by the cooler overlying atmosphere. 

Each atom or molecule will emit or absorb energy of one or more colors. 
It is known from laboratory studies what colors are emitted or absorbed by 
a particular element or compound. To detect this element or compound, all 
you have to do is see the appropriate color or colors being emitted or absorbed 
in the infrared data collected by IRIS. 

Using this procedure, IRIS has detected hydrogen, helium, water, 
methane, acetylene, ethane, ammonia, phosphine, and germane in the at- 
mospheres of Jupiter, Saturn, and Uranus. 

Ultraviolet Spectrometer (UVS) 

The UVS is also a very specialized type of light meter that is sensitive 
to ultraviolet light. “Ultraviolet” means more than or beyond violet. Ultra- 
violet light is next to and above (in frequency) the 
1989/08/25 2:10 GMT (SCT) violet light that our eyes can see. Sunburns and 

suntans are caused by ultraviolet light. 

The UVS is used to determine when certain 
atoms or ions (electrically charged atoms) are pres- 
ent, or when certain physical processes are going on. 


TO NEPTUNE: 139465 KM 

It works on the same physical principle as the IRIS. However, instead of 
using a lens, the UVS limits the area of sky it looks at by using a series of 
“blinders” called aperture plates. The UVS looks for specific colors of 
ultraviolet light that certain elements and compounds are known to emit or 

The Sun emits a large range of colors of light. If sunlight passes through 
an atmosphere, certain elements and molecules in the atmosphere will 
absorb very specific frequencies of light. If the UVS, when looking at filtered 
sunlight, notices the absence of any of these specific colors, then particular 
elements and/or compounds have been detected. This process is called 
identifying elements or compounds by atomic absorption (Figure 5-2). 

The UVS can only use the atomic absorption technique when it is in a 
position to look back at the Sun (or a suitably bright star), through a 
planetary or satellite atmosphere or through a collection of ring particles. 
This geometry is called a solar (or stellar) occultation. 

The UVS has used these emission and absorption techniques to detect 
hydrogen, helium, methane, ethane, acetylene, sodium, sulfur, nitrogen, 
and oxygen. The UVS, like IRIS, has been used to detect most of the gases 
and their photochemical products found in the atmospheres of the giant 
planets, though at much higher levels in the atmosphere. 

Figure 5-2. Certain molecules in the atmosphere can absorb particular wavelengths 
from the Sun's energy , and the UVS can spot these missing “lines.” 


Under certain conditions, the UVS is sensitive to energy that is emitted 
when lightning occurs, when an auroral display is going on, or from particles 
independently orbiting the planets. 

The UVS can also be used to study the stars. It can determine when 
certain elements are present in various stars and measure the temperatures 
of extremely hot stars. The UVS instruments on both Voyagers have been 
used for years as stellar observatories because they can see colors completely 
blocked by the Earth’s atmosphere. The UVS is making fundamental 
contributions to ultraviolet astronomy. 

Photopolarimeter Subsystem (PPS) 

The PPS is the fourth of the specialized light-measuring devices on 
board Voyager. The PPS is very much like the ISS narrow-angle camera in 
that it has a very high magnification reflecting telescope. It is unlike the ISS 
narrow-angle camera in that each PPS measurement produces one picture 
element (pixel), whereas each ISS image consists of 800 lines, with each line 
consisting of 800 pixels. Of the Voyager science sensors that are primarily 
sensitive to visible light (the two ISS cameras, the IRIS radiometer, and the 
PPS), the PPS is by far the most sensitive. 

The PPS allows the most flexibility in adapting to varying circum- 
stances. It has four aperture settings, three color filters, and four polarizing 
filters. It also has two commandable sensitivities. 

The PPS studies how light changes as it is reflected from or absorbed 
by objects of interest. Such “objects” include the surfaces of moons, tiny 
aerosol particles in an atmosphere, and ring particles. The PPS can infer the 
texture and composition of a solid surface; the density, particle sizes, and 
composition of a planetary ring; and the existence, particle sizes, and 
composition of atmospheric hazes. 

The PPS is ideally suited for observing stellar occultations (Figure 
5-3). The PPS is sensitive enough to light to be able to track a star as it moves 
behind planetary rings or the thin part of a planetary or satellite atmos- 
phere. Planetary rings are a collection of countless small objects in orbit 
about a planet. Since there is space between the ring particles, light can pass 
through the rings. The PPS is used to record the rapid 
variation of brightness from a star as it passes behind 
a set of rings. The fraction of starlight that passes 
through is an indication of how “transparent” the 
rings are and where gaps are located. The PPS stellar 


133839 KM 


Figure 5 - 3 . Like watching a flashlight moving behind a partially transparent picket 
fence , the PPS can accurately measure the amount of starlight passing through a 
planetary ring system. 

occultation technique produced a bounty of information about the complex 
structure of the rings of Saturn and Uranus. 

Radio Science Subsystem (RSS) 

All of the sensors on the spacecraft, except for the RSS, are called 
passive sensors. A passive sensor must wait for energy or particles to come 
to it before it can see anything. A passive sensor must have another source 
create the energy or particles that it sees. The RSS is an active sensor. An 
active sensor provides both its own energy and the detector to measure the 
effect of the target of interest on the energy. 

The energy source for the RSS is the same radio transmitter that is used 
for communications between the spacecraft and the Earth. The Radio 
Science detectors are located at Deep Space Network (DSN) tracking 
stations on Earth. The RSS is capable of transmitting stable carrier 
frequencies at both S- andX-band using an Ultra-Stable Oscillator (USO) on 
board the spacecraft. To achieve even more stable carrier frequencies, the 
RSS can receive and retransmit an extremely precise signal sent from the 
DSN antennas. Four distinct types of experiments have been performed 
using the RSS. 

The RSS is used to probe both planetary and satellite atmospheres. 
When the spacecraft is passing behind a planet or a moon with an atmos- 
phere, the spacecraft’s radio signal is beamed through that atmosphere. The 
signal will be bent and slowed by the atmosphere by a process called 



Figure 5-4. The Radio Science Subsystem uses the spacecraft's radio transmitter to 
beam a signal through a body's ionosphere and neutral atmosphere for detection by 
the DSN antennas on Earth. The signal changes allow an estimate of the density, 
temperature , pressure, and composition of the atmosphere. 

refraction. The spacecraft antenna is pointed such that the bent radio signal 
reaches the Earth (Figure 5-4). This viewing geometry is called an Earth 

From the changes in the frequency and the intensity of the X- and S- 
band signals, the temperature, pressure, density, and composition of the 
atmosphere at different altitudes can be calculated. If an ionosphere exists 
outside the atmosphere, it will also change the radio signal. Studies of the 
corona of the Sun are car ried out in a similar fashion when the spacecraft 
passes behind or near the Sun as viewed from Earth. 

The second type of RSS experiment involves directing the X-band radio 
signal through planetary rings. When the spacecraft is behind a set of 
planetary rings (from an Earth perspective), the antenna is pointed directly 
at the Earth. The signal passes through the rings and is received on the 
Earth. Changes in the X- and S-band signal intensi- 
ties and frequencies can be used to estimate the 
number, width, shape, and thickness of rings, and the 
sizes of the particles that make up rings. 

The third type of RSS experiment is the determi- 
nation of the mass of a planet or moon that the 
spacecraft passes at close range. This experiment 

TIME: -0 Days. 1:40:00 
1989/08/25 2:20 GMT (SCT) 


128202 KM 


works on the Doppler principle. The spacecraft’s radio transmitter sends a 
signal at a well-known stable frequency. Any change in speed (acceleration) 
that the spacecraft experiences will cause the frequency of the radio signal 
received by DSN antennas on Earth to change. The amount of frequency 
change (the Doppler shift) is dependent on the change in speed of the 
spacecraft, relative to Earth. 

When the spacecraft passes close to a planet or moon, that body pulls 
on the spacecraft, causing its speed to increase during approach and to 
decrease during departure. The amount of change in speed depends only 
upon the mass of the planet or moon and the distance of the spacecraft from 
the planet or moon. Thus, by measuring the change in frequency of the 
Earth- received radio signal, the mass of the planet or moon can be estimated. 

The final RSS experiment is a test of general relativity. The Theory of 
General Relativity predicts an apparent slowing in the speed of a radio signal 
when the signal passes near any massive body. Each time the Voyager 
spacecraft signal passes close to the Sun (the most massive solar system 
body), the radio signal is delayed. This observed delay is compared to that 
predicted by the Theory of General Relativity. So far, general relativity has 
passed the test each time. 

Fields , Waves , and Particles Experiments 

The fields, waves, and particles experiments are organized as six 
science investigations. The Planetary Radio Astronomy (PRA) Subsystem is 
designed to measure radio waves from the Sun and planets. The Plasma 
Wave Subsystem (PWS) detects radio waves generated near the spacecraft. 
The Magnetometer (MAG) Subsystem measures the strength and orienta- 
tion of magnetic fields through which the spacecraft is passing. 

The remaining science investigations measure charged particles of 
various energies. They include the Plasma Subsystem (PLS), the Low- 
Energy Charged Particle Subsystem (LECP), and the Cosmic Ray Subsys- 
tem (CRS). 

Planetary Radio Astronomy (PRA) 

The PRA is a sophisticated radio receiver, attached to a pair of 10-m 
(33-ft) “rabbit ears”. The PRA listens for radio signals produced by the Sun 
and the planets, their magnetospheres, and lightning. The Jovian, Satur- 
nian, and Uranian systems all produced such signals. 

The PRA works in one of two ways. Suppose you had three favorite 
radio stations, but that you were a compulsive button pusher. As the PRA 
switches among three single (programmable) frequencies, dwelling on each 


for only six seconds, it would be like pushing your favorite radio buttons 
every six seconds. 

Consider, however, a second case. Suppose you knew that there was 
going to be a college football game broadcast at 1 PM on Saturday, but you 
didn't know the station. You would start your radio receiver at the extreme 
left end of the radio dial, and sweep across. You would stop at each station 
long enough to recognize the signal. If it wasn't the game, you would pass 
on to the next station. If none of the stations were broadcasting the game 
(perhaps because it was on FM and you are tuning the AM band), you would 
have surveyed the entire band. This is how the PRA works in its scanning 
mode, except that it starts at the higher frequencies and sweeps down. 
Because it senses signals from each of its “rabbit ear” antennas separately, 
it can also detect the polarization (vibration pattern) of the radio waves. 

Plasma Wave Subsystem (PWS) 

The PWS, like the PRA, is essentially a radio receiver and amplifier. It 
listens for signals at frequencies that the human ear could hear (audio 
frequencies), as well as at frequencies slightly above audible. The PWS 
shares the 10-m pair of rabbit ears with the PRA, but uses them as a single 
antenna. With an effective length of only 7 m (23 ft) the PWS normally 
operates in a scanning mode. If you tuned your radio receiver first to one 
station, then another, then another, up to sixteen stations, you would be 
operating your receiver as the PWS does when it is scanning. 

The PWS has a second mode of operation. It can simultaneously listen 
to all the stations on its audio band. This mode is used most frequently when 
the spacecraft is near a planet, and can operate simultaneously with the 
scanning mode. 

The PWS samples the behavior of plasmas in and around planetary 
magnetospheres by measuring the radio waves generated by those plasmas. 
It can also detect planetary lightning and the presence of tiny ring particles 
that strike the spacecraft while it moves through the plane of the rings. 

Magnetometer (MAG) 

TIME :-0 Days. 1:35:00 
1989/08/25 2:25 GMT (SCT) 

Although the MAG can detect some of the effects 
of the solar wind on the outer planets and moons, its 
primary job is to measure changes in the Sun’s mag- 
netic field with distance and time, to determine if each 
of the outer planets has a magnetic field, and how the 
moons and rings of the outer planets interact with 
those magnetic fields. If it detects a planetary mag- 



122556 KM 

netic field, its job then becomes to measure the characteristics of the 
magnetic field. 

The MAG can be visualized as four sets of compasses. Each set consists 
of three compasses, mounted at right angles to each other. Two of the 
compass sets are very sensitive, and can detect magnetic field strengths as 
weak as 1/10,000 the strength of the magnetic field of the Earth’s surface. 
The other two sets are not nearly so sensitive, and are designed to detect 
large magnetic field strengths, some 20 times stronger than the Earth’s 
magnetic field. To avoid detecting magnetic fields generated by the space- 
craft itself, the more sensitive compasses are mounted twenty feet out and 
forty feet out on a long boom. 

Particle Detectors 

The PLS, LECP, and CRS all detect the impact of electrically charged 
particles. The difference between the three sets of sensors is that the PLS 
basically looks for the lowest-energy particles, and the LECP and CRS look 
for higher-energy particles. All three sets of sensors work by sensing 
particles that hit them. 

Plasma Subsystem (PLS) 

A “plasma” is a gas or “soup” of charged particles. It typically consists 
of electrons as well as positively charged nuclei produced when the original 
atoms lost one or more of their electrons. The PLS looks for the lowest-energy 
particles. It also has the ability to look for particles moving at particular 
speeds and, to a limited extent, to determine the direction from which they 
came. The PLS can be imagined as a piece of wood with a variable amount 
of syrup in front of the wood. The syrup slows the particle down so that it can 
just hit the wood and stick to it. Up to a point, if one wants to look for faster 
particles, one simply puts more syrup in front of the wood. In actuality, the 
PLS steps through various amounts of “syrup” looking for particles moving 
at various speeds. 

Low-Energy Charged Particle (LECP) and Cosmic Ray Sub- 
system (CRS) 

The LECP and CRS look for particles of higher energy than the PLS, at 
overlapping energy ranges. The LECP has the broadest energy range of the 
three sets of particle sensors. The CRS looks only for very energetic particles, 
and has the highest sensitivity. Both of these sensor sets also have a limited 
ability to determine the particle directions and compositions. 


The LECP can be imagined as a piece of wood, with the particles of 
interest playing the role of bullets. The faster a bullet moves, the deeper it 
will penetrate the wood. Thus, the depth of penetration measures the speed 
of the particles. The number of "bullet holes" over time indicates how many 
particles there are in various places in the solar wind, and at the various 
outer planets. The orientation of the wood indicates the direction from which 
the particles came. 

The CRS looks for particles with the highest energies of all. Very 
energetic particles can often be found in the intense radiation fields sur- 
rounding some planets (like Jupiter). Particles with the highest-known 
energies come from other stars. The CRS looks for both. 

The CRS makes no attempt to slow or capture the super-energetic 
particles. They simply pass completely through the CRS. However, in 
passing through, the particles leave signs that they were there. Thus, the 
CRS can be visualized as simply a piece of wood that the bullets pass 
completely through. One simply counts the “bullet holes”, to know when a 
high-speed particle impacted, and how often. The CRS has seven separate 
“pieces of wood” oriented in different directions, and looking for different 
types of high-energy particles. 

Sensor Engineering Characteristics 

Figure 5-5 provides a brief summary of the engineering characteristics 
of each of Voyager’s eleven scientific investigations. 

Prior to launch, an attempt was made to perfectly align the fields of 
view of the optical instruments when they were mounted on the scan 
platform. This enables them to provide complementary information as they 
all view the same target. As you might imagine, it is not easy to align 
relatively bulky electronic equipment to ultra-high precision. However, as 
shown in Figure 5 - 6 , the alignment mechanics did a first-class job, being 
within 0 . 1 ° of their objective. 

The Physics of the Optical Target Body Instruments 

If you’re still with us, you must be quite interested in how the Voyager 
sensors help us learn about other worlds. So now is 

TIME:-0 Days, 1:30:00 „ , . c 

1989/08/25 2:x gmt (sct) the time for your physics refresher, to tie oft some of 

the basic concepts. The Voyager optical sensors that 
actually point at something (the ISS, IRIS, UVS, and 
PPS) all work on the same basic principles of atomic 


TO NEPTUNE: 116900 KM 


The Imaging Science Sub- 
system consists of two 
television-type cameras, 
each with 8 filters. One has 
a 200 mm wide-angle lens 
with an aperture of f/3, while 
the other uses a 1500 mm 
narrow-angle f/8.5 lens. 

The Photopolarimeter Sub- 
system uses a 0.2 m 
telescope fitted with filters 
and polarization analyzers. 
It covers eight wavelengths 
in the region between 235 
^m and 750 urn. 

The Infrared Radiometer 
Interferometer and 
Spectrometer measures 
radiation in two regions of 
the infrared spectrum, from 
2.5 to 50 jim and from 0.3 to 

2.0 jim. 

The Ultraviolet Spectrometer 
covers the wavelength range 
of 40 jim to 1 80 jim looking at 
planetary atmo- spheres and 
interplanetary space. 


The Plasma Subsystem 
studies the properties of 
very hot ionized gases that 
exist in interplanetary 
regions. One plasma 
detector points in the 
direction of the Earth and 
the other points at a right 
angle to the first. 


The Low-Energy Charged 
Particle experiment uses two 
solid-state detector systems 
mounted on a rotating plat- 
form. The two subsystems 
are the low energy particle 
telescope (LEPT) and the low 
energy magnetospheric 
particle analyzer (LEMPA). 

The Cosmic Ray Subsystem 
detector measures the energy 
spectrum of electrons and 
cosmic ray nuclei and uses 
three independent systems: a 
high-energy telescope system 
(NETS), a low-energy 
telescope system (LETS), and 
an electron telescope (TET). 


The Magnetic Fields experi- 
ment consists of four magne- 
tometers: two are low-field 
instruments mounted on a 
10-m boom away from the 
field of the spacecraft, while 
the other two are high-field 
magnetometers mounted on 
the body of the spacecraft. 

PWS and PRA 

Two separate experiments, the Plasma Wave Subsystem and the 
Planetary Radio Astronomy experiment, share the two long 
antennas which stretch at right-angles to one another, forming a "V". 
The PWS covers a frequency range of 10 Hz to 56 kHz. The PRA 
receiver covers two frequency bands, from 20.4 kHz to 1300 kHz 
and from 2.3 MHz to 40.5 MHz. 


The investigations of the Radio Science Subsystem are 
based on the radio equipment which is also used for 
two-way communications between the Earth and Voyager. 
For example, the trajectory of the spacecraft can be 
measured accurately from the radio signals it transmits; 
analysis of the flight path as it passes near a planet or 
satellite makes it possible to determine the object’s mass, 
density and shape. The radio signals are also studied at 
occultations for information about atmospheres, 
ionospheres, and ring particles. 

Figure 5-5. Each Voyager spacecraft carries a science payload of 110 kg (240 lb) that 
uses 100 watts of power. Eleven investigations are designed to return complementary 
science across a broad spectrum. 


Figure 5-6. An attempt was made to align (for complementary science) the 
viewing axes of the seem platform optical instruments before launch , but 
slight misalignments of up to 0.1 ° were unavoidable when mounting fairly 
bulky instrument packages (Voyager 2 shown). 

All matter in the universe above absolute zero temperature is under- 
going some type of atomic (including molecular) motion. In fact, temperature 
is nothing more than the measure of the average amount of motion of a group 
of atoms or molecules. Matter that is hotter (higher in temperature) simply 
has faster-moving atoms or molecules. 

When an atom or molecule absorbs or releases energy, it may do so only 
in certain ways. The central (and founding) idea of quantum mechanics is 
that matter may absorb or release energy only in “chunks” or quanta. This 
idea was conceived of and published by Max Planck in 1 900. Five years later, 
Albert Einstein extended the idea by postulating that energy itself can only 
exist in quantized amounts, i.e., in integer multiples of some fundamental 
amount of energy. These two ideas form the intellec- 
tual starting point for quantum mechanics. 

We have been loosely using the term “atomic 
motion,” but we can see in Figure 5-7 that we must 
visualize three distinct types of motion. Molecules, 
consisting of two or more atoms, may rotate and 

TIME: -0 Days. 1:25:00 

1989/08/25 2:35 GMT (SCT) 


111234 KM 







Low Energy 
Low Frequency 
Long Wavelength 
(Radio, Microwave, 
Far Infrared) 




(' 0 

Medium Energy 
Medium Frequency 
Medium Wavelength 
(Near Infrared) 




Key Equations: C = f X, E = hf 

High Energy 
High Frequency 
Short Wavelength 
(Visible Light, 

Figure 5-7. All matter above absolute zero temperature is constantly in motion , 
absorbing and re-emitting energy. Molecules , consisting of two or more atoms , can 
possess rotational and vibrational energy , as well as electron orbit transfers. Single 
atoms typically possess only the orbital transfers of electrons. 

vibrate at different discrete energy levels. A molecule may absorb a specific 
quantum of energy, thereby moving to a higher energy state, or it may 
release a quantum of energy in the form of an electromagnetic wave. The 
lowest energy state changes are associated with changes in the rotational 
energy of the molecule. 

Both molecules and individual atoms can undergo relatively large 
energy state changes whenever their “orbital” electrons jump between 
specific “orbits,” again experiencing discrete energy-level changes charac- 
teristic of the particular molecule or atom in question. As shown in both 
Figures 5-7 and 5-8, these electron “orbit” transitions result in greater 
energy-state changes than those associated with changes in molecular 
rotation and vibration states. 

Because each change in energy level is unique, a particular type of atom 
or molecule may be identified either when light (more generally, an electro- 
magnetic wave) of a frequency it is known to emit is present, or when light 
of a frequency it is known to absorb is absent. The former is known as atomic 
or molecular emission. The latter is known as atomic or molecular absorp- 
tion. Both techniques are used by the Voyager optical target body sensors to 
identify the presence of particular atoms and molecules. 

The frequencies of light that a specific diatomic or polyatomic molecule 
can absorb or emit can be calculated. By determining, in advance, the atoms 




Figure 5-8. Molecules and atoms can only absorb and re-emit discrete packets of 
energy. The exact values of these energy level changes are unique to different 
molecules and atoms , acting like fingerprints to identify the matter in question. 

and molecules that one wants to locate, one can determine the range of 
electromagnetic frequencies that must be detectable. Once the desired range 
of frequencies is known, one can design sensors that will indicate the 
presence of a particular atom or molecule. 

Figure 5-9 shows both the range of wavelengths that the Voyager 
optical target body sensors can detect, and some of the 
atoms and molecules that emit or absorb energy 
within this wavelength range. One can see the large 
number of molecules potentially detectable by the 
IRIS; hence, the importance of preserving the health 
of the instrument for Neptune. 

TIME: -O Days. 120:00 
1989/08/25 2:40 GMT (SCT) 


105561 KM 


r "" PP \^1 

D I I 





10 "' 




C 2 Hg (ETHANE) 


I I I 

(AMMONIA) NH 3 >_ 

(ETHYLENE) C 2 H 4 ^^ 

„ C 3 H 8 (PROPANE) 



10 " 





Figure 5 - 9 . Voyager's optical instruments cover a large range of the electromagnetic spectrum , 
enabling the detection of many possible substances. 

By now, you must be ready to dive into the Neptune encounter 
activities, having been briefed on the mission and science basics in Chapters 
1 to 5. As the Guide will be published during the early portion of the 
Observatory Phase, we should hurry on to Chapter 6 in order to learn all 
about the exciting Neptune encounter plans and highlights. 

TIME :-0 Days. 1:15:00 
1989/08/25 2:45 GMT (SCT) 



99881 KM 

Everything comes to those who can wait . 

Francois Rabelais 


It’s been nearly twelve years since Voyager 2 was sent on its Grand 
Tour of the outer solar system, and the last planet on the trip, Neptune, is 
just ahead. For the past three and one-half years, the bulk of the Voyager 
Project's efforts have been focused on planning and designing an extensive 
four-month, four-phase Neptune encounter — one which promises to be 
perhaps the most exciting of the Tour. 

Table 6-1. The four phases of the Voyager-2 Neptune encounter activity span 
nearly four months. Most of the highest value science is taken during the NE phase. 

Encounter Phase Start and End Dates Length 



Jun 05 


Aug 06 



Far Encounter 


Aug 06 


Aug 24 



Near Encounter 


Aug 24 


Aug 29 



Post Encounter 


Aug 29 


Oct 02 



Before we get into the detailed discussion of activities planned for the 
actual encounter period (see also Table 11-1), it would be helpful first to run 
through a brief orientation. Maintaining a mental image of where the action 
is happening and what’s going on is key to understanding the encounter 
design. Once oriented, a description of what it takes to plan a visit like this — 
to a place our species has never been before — might help you form an 
appreciation for the behind-the-scenes work required to pull it off. It has 
been a first-rate challenge. 

Getting Our Bearings 

What little we know about this remote destination was summarized in 
Chapter 2. Chapters 3, 4, and 5 brought you a bit closer to the hearty Voyager 
spacecraft and the persevering people that got it to where it is now. But the 
solar system is a big place, and maybe you need a little help to see in your 
mind’s eye where all of this is happening. 

Just where are we going, anyway? Where is Neptune? Can someone 
point to Voyager 2? Let’s think about these questions for a while and get our 


bearings. We’ve traveled far from home the past twelve years, so a check of 
the map might be a good idea about now. 

Take another look at Figure 1-4, our “bird’s eye view” of the solar 
system. Note first that, on the scale shown, the Earth is pretty close to the 
Sun. In fact, they’re just a few degrees apart from Voyager’s point of view. 
Now follow Voyager 2 on its Grand Tour path from Earth, past Jupiter, 
Saturn, and Uranus, and on to Neptune. Far from home, indeed! From 
Earth, Neptune is 4.5 billion km (2.8 billion mi) as the crow flies (or in this 
case, as the radio signal propagates). This is thirty times farther than the 
distance from the Earth to the Sun! 

But that’s not all: if Voyager 2 had an odometer on board, it would read 
nearly 7 billion km (nearly 4.4 billion mi) upon arrival at Neptune, since the 
more scenic Grand Tour route is longer than the straight-line distance. Our 
chosen path to Neptune is by no means the shortest way there, but it is the 
most practical path, which is what really counts in the space travel business. 
Chapter 7 explains this in detail. 

Now let’s get something else straight. How is Voyager 2 approaching 
Neptune? Refer to Figure 1-4 again, and note which way the planets circle 
the Sun. Voyager 2’s path (trajectory) from Uranus to Neptune has been 
going away from the Sun. but not straight away: it has a moderate compo- 
nent in the same direction as Neptune’s motion around the Sun as well. This 
trajectory does not actually aim Voyager 2 for Neptune's apparent location, 
but for where we believe Neptune will be at a precise time in late August; 
until that time, the planet will be shy of this aiming point, creeping slowly 
towards it every day. The most exciting part of the encounter is when this 
huge gaseous body gets buzzed by our tiny spacecraft when they meet at the 
aiming point. 

The relative positions and velocities between Voyager and Neptune as 
they race to this aiming point create a useful viewing geometry. The 
spacecraft velocity component in the same direction as Neptune’s orbital 
motion is nearly the same magnitude, too: 468,000 km/day, or 290,000 mi/ 
day. These matched speeds keep either body from passing the other during 
most of the approach. In fact, the relative positions between the two are such 
that our spacecraft tends to stay close to the Sun- 
Neptune line, which gives its cameras and other 
sensors a nearly “full-moon” view of anything in the 
Neptunian system — the planet, Triton, Nereid, and 
other possible moons we haven’t even discovered yet. 
(The reason we don’t see a complete full-on view is 
because Voyager 2 is not exactly on the Sun-Neptune 


TIME: -0 Days. 1:10:00 
1989/08/25 2:50 GMT (SCT) 

line; a thin crescent of darkness shows on the right-hand side of the planet 
and moons as a result of this offset.) These high-illumination lighting 
conditions give us our brightest view of the objects there, and optimize our 
chances for discovering new moons long before Voyager visits them. 

What about the other velocity component, heading essentially along 
(but not exactly on) the Sun-Neptune line? This component is over three 
times as fast as the other, and brings Voyager 2 closer to Neptune at the rate 
of 1.45 million km/day (900,000 mi/day). This speed would take Voyager 
from the Earth to the Moon in less than seven hours! At such a pace, it’s no 
wonder Neptune gets visibly bigger day by day in the pictures beamed back 
during approach. 

So, Voyager 2's approach to Neptune is easy to visualize if you just 
remember where you are standing. Stand at the aiming point, and both 
Voyager and Neptune will appear to be racing towards you on a collision 
course. Stand near Neptune, and Voyager will appear to be heading straight 
for you, coming right out of the Sun like an attacking fighter. Take our view 
from Earth, and you will see both bodies sweeping along at the same rate, 
with Voyager closing in on Neptune in a direction almost straight away from 

Now you know how far away Voyager 2 is from us and generally how 
it is approaching Neptune. But where is Neptune? With Neptune’s position 
known, one might guess (correctly) that Voyager 2 would be nearby. Given 
this, we can then impress our friends and actually point at our distant 
spacecraft. A visualization of the layout of the solar system will help us 
figure this one out. 

The planets, the asteroids, and many comets lie approximately in the 
same plane as they orbit the Sun. Astronomers keep track of the small 
differences between these similar planes by referencing them to the orbit 
plane we’re most familiar with: the Earth’s. This plane is known as the 
ecliptic plane; in the context of Figure 1-4, it would be the sheet of paper it's 
printed on. 

From our viewpoint on Earth’s surface, these bodies (as well as the Sun) 
appear to rise at roughly the same place in the east (but at different times), 
traverse up into the sky along an arc, and set at about the same place in the 
west, again at different times. This track that the train of objects seems to 
be chugging along on is simply our edge-on view of the ecliptic plane, and 
their rise-and-set behavior we observe is caused by the turning of Earth on 
its axis. The planets, asteroids, and comets also move against the fixed starry 
background from night to night — moving through the signs of the Zodiac, as 
we say — but this motion is trivial compared to the rise and set motion. 


Now let’s visualize where Voyager 2 is again. It started at Earth, 
visited Jupiter, Saturn, and Uranus, and is now relatively close to Neptune; 
all are residents of the ecliptic plane, including Voyager 2. We now have 
enough clues to figure out where to point our finger. 

So where is our remote explorer? Step outside on a clear night and 
mentally draw the arc that passes through that day’s sunrise and sunset 
points on the east and west horizons, respectively, and one or more of the 
visible planets (Mercury, Venus, Mars, Jupiter, or Saturn). This arc is the 
current edge-on view of the ecliptic plane. Then, on this arc, find — or have 
a friend that knows the stars help you find — the constellation Sagittarius, 
which appears near the eastern edge of the Milky Way. In that region of the 
sky Neptune is imperceptibly marching in its 165-year orbit from west to 
east along the ecliptic arc. Just a fraction of a degree more to the west, an 
aging, yet accomplished, spacecraft is closing in fast. 

Not even the strongest telescope on Earth can see Voyager 2, but we 
know it’s there, because we can hear it. And, after all these years, it can still 
hear us. In spite of the long wait, Voyager 2 is ready for Neptune, and 
so are we. 

Aiming for Neptune 

So . . . we’re ready for a tour of the Neptunian system and our des- 
tination looms larger ahead day by day. Where do we want Voyager 2 to go, 
exactly? We don’t get to pass through this distant place very often, and with 
so many tantalizing sights to train Voyager’s sensors on, we would like it to 
be at all the right places at all the right times. Chapter 2 highlighted the 
interest in the large moon Triton. In addition to a close pass by Triton, we 
would also like Voyager to get close to Neptune and well into its magneto- 
sphere, passing behind the planet if possible to ensure that the spacecraft's 
sensors and the Deep Space Network (DSN) antennas on Earth can take 
advantage of the high-value Earth and Sun occultation conditions that such 
a trajectory produces. Occupations by Triton would be great to have as well. 
A variety of views of the purported ring-arc system would also be welcome, 
with one or two bright star occupations if possible. But in our zest to gather 

TIME: -0 Days, 1:05:00 
1989/08/25 2:55 GMT (SCT) 

all of this exciting science, we should not endanger 
our vulnerable spacecraft by placing it too close to the 
potential hazards posed by the ring-arc system, at- 
mosphere, and radiation belts (see Chapter 8). In 
short, we are presented with a virtual smorgasbord of 
science, and we would like Voyager to gulp in as much 
as it (and we) can handle. 



88514 KM 

In this regard, Figure 6-1 is offered to whet your appetite. It shows the 
path Voyager 2 will take through the Neptunian system — one which satis- 
fies nearly all of our science desires without taking on undue mission risk. 
(For reference, this view is close to what you would have if standing on the 
surface of far-away Nereid during the encounter.) Let’s investigate why 
this trajectory was selected. 

Although the success of this encounter depends on dozens of critical 
decisions, among the most important is the selection of the aiming point — 
exactly where in space and time Voyager 2 will come closest to Neptune. The 
aiming point sets up the geometry and timing for all encounter events, and 
thus controls most of the science-related parameters. 

North Pole 


Voyager 2 at 

Earth and Sun 


Earth and Sun 

Voyager 2 at 

Ring- Arc 

Triton Sun 
and Earth 



0 50 100 150 200 

Thousands of km 

Figure 6-1 . From this perspective, Neptune is coming at you out of the paper, and 
the Voyager-2 trajectory is in the plane of the paper. Note the position of Triton, 
and the gravitational bending of Voyager's trajectory required to reach it by diving 
closely over the northern polar regions of Neptune. 


An added bonus (and complication) at Neptune is that, for the first time 
on the Grand Tour, we are free to target Voyager 2 to any reasonable aiming 
point. At Jupiter, Saturn and Uranus, we had to worry about getting to the 
next planet, so rigid “swingby corridor” aiming-point constraints were 
imposed to comply with the Grand Tour trajectory design requirements. 
These constraints simplified the aiming-point selection process, and served 
their purpose well; they are chiefly responsible for guiding Voyager to 

Incidentally, you may be wondering why we don’t send Voyager 2 
towards Pluto after the Neptune encounter. After all, we have never been 
there, and being so close now, it seems to be a waste not to give it a try. It 
turns out that there is an aiming point at Neptune that could gravity-assist 
Voyager to Pluto, but there’s a big problem with it: it requires such a close 
pass by Neptune that the spacecraft would actually have to go deep into the 
planet itself! Clearly, this option is not practical. Pluto will have to wait. 

Thus, here we are, with a multitude of aiming points from which to 
choose. Among these possibilities, might one class satisfy our desires more 
than the others? After much study and analysis, the scientists and encoun- 
ter designers have concluded that the answer to this question is yes, for 
several reasons. 

The aiming point selection process for Neptune proved to be an arduous 
task, and was certainly one of the most challenging of all the Voyager 
encounters, including those for Voyager 1 . A discussion of the detailed steps 
of this process is beyond the scope of this Guide, so only the highlights of this 
eight-year exercise (1980-1988) will be treated here. 

To visualize the aiming-point possibilities that were considered in 1 980 
(before Voyager 2 had even arrived at Saturn!), imagine a scene where a 
huge sheet of thin paper thousands of kilometers on a side is placed 
perpendicular to the line connecting Voyager and Neptune on the space- 
craft’s post-Uranus trajectory, like a huge billboard between the two. (Recall 
that from Neptune’s point of view, Voyager is heading straight for it, and 
thus straight for this billboard.) Think of this sheet as a big target that the 
Voyager navigators want to shoot the spacecraft through, using Voyager’s 

1989/08/25 2:60 GMT (SCT) 

TIME: -0 Days, 1:00:00 

maneuvering capability. Shoot Voyager through a 
different spot on the sheet, and you get a different set 
of encounter conditions (geometry and timing) at 
Neptune. Figure 6-2 shows what we would want to 
draw on this target to help the Navigation Team with 
their shot. 



82834 KM 

The circular region of aiming points on our target leads to a collision 
with Neptune, so we certainly don’t want to shoot anywhere in there. This 
holds even for point P, which would send us on to Pluto . . . but for the fact 
that we cannot shrink Neptune’s mass to a point to allow the math to be 
realized. At the far right side of Figure 6-2, near the point labeled N 1? a line 
of aiming points results in a collision with Triton, as does a small arc of points 
in the northern polar area of the planet. Targeting just to the side of either 
arc would give us an arbitrarily close pass by Triton, with the possibility of 
Earth and Sun occultations by Triton in some cases. The Earth and Sun 
occultations by Neptune are a bit easier to get: the acceptable aiming points 
map into a broad region across the target (mostly because Neptune is so big). 

If we really want that close pass by Triton, we have to make a choice. 
A point near N x passes far from Neptune; this choice satisfies Triton and 
Neptune occupation science objectives and safety concerns, but compro- 
mises other Neptune science investigations that would work better if done 
closer to the planet. In contrast, close-in aiming points in Neptune’s 
northern polar region satisfy all major science objectives (including Triton 
occultations), but introduce some concerns about environmental hazards. 

Intermediate aiming points such as that labeled N 2 are also available 
if one wants to sit on the fence and be safe about everything, but the result 
is a dull encounter — Voyager wouldn’t get very close to Neptune or Triton. 

To make the first part of a very long story short, the northern polar 
region, near the “inner Triton locus," was judged in 1980 as the most 

T-Axis, 10^ km 

Figure 6-2. Our giant target between Voyager 2 and Neptune is shown here , marked 
with the Neptune aiming-point possibilities that were studied over eight years ago. 


desirable place to pass by Neptune. (The ring arcs, incidentally, weren’t 
discovered until late 1980, after this original aiming point had been se- 
lected!) This region requires a close pass over the northern polar region of 
Neptune to gain sufficient gravitational deflection (see Figure 6-1 ) for a close 
encounter with Triton five hours later. Properly executed, such a trajectory 
could please nearly all members of the Voyager science, engineering, and 
management communities. After considerable study, an original aiming 
point resulting in a 44,000 km (27,300 mi) Triton miss distance was chosen, 
with an associated Neptune arrival time (closest approach) of 23:12 
Greenwich Mean Time (GMT) on August 24, 1989. 

As Neptune encounter planning progressed after this original aiming 
point was selected, the Voyager Imaging Team saw the need for a closer 
Triton pass to reduce the level of smear in the Triton images. (A closer pass 
causes the spacecraft to take a more direct, head-on approach to Triton, 
reducing the cross-wise motion of the moon in Voyager’s cameras.) In 
addition, the Radio Science Team asked for a slightly later arrival time to 
bring Voyager 2 higher above Australia’s horizon (by virtue of Earth’s rota- 
tion) for the important Earth occultation experiments at Neptune and 

These requests were approved in the fall of 1985, leading to a new 
Triton miss distance of 10,000 km (6200 mi) and a Neptune closest approach 
time of 04:00 GMT on August 25. This new trajectory was judged to be the 
best for science, while still keeping outside the environmental hazard zones, 
including the newly discovered ring arcs. 

Then, in late 1985, the estimates for four separate parameters describ- 
ing the Neptunian system were updated based on the latest available 
knowledge, and all four changes caused an increased environmental con- 
cern. In fact, the nominal aiming point appeared — on paper — to send the 
spacecraft straight through the postulated ring-arc system and dangerous- 
ly close to the atmosphere! The Project was sent into high gear reevaluating 
the encounter design, and the trajectory had to be moved out. By late 1986, 
a new aiming point was baselined that retained the 04:00 Neptune arrival 
time, but moved the Triton miss distance out to 40,000 km (25,000 mi). 

TIME: -0 Days. 0:55:00 
1989/08/25 3:05 GMT (SCT) 

Figure 6-3 zooms in on Neptune’s northern 
polar region from Figure 6-2 and depicts our present 
aiming space. The latest physical models of the 
Neptunian system suggest that the “40K Triton” 
aiming point is safe with at least 95 percent confi- 
dence, probably more. Voyager 2 is expected to pass 
about 5000 km (3100 mi) outside the hazard thresh- 



77167 KM 

T-Axis, 10^ km 

Figure 6-3. The northern polar aiming space at Neptune satisfies all major science 
objectives , but introduces some environmental hazard zones to be given safe 

olds for both the ring-arc system and atmosphere, and will probably be 
exposed to measurable — yet tolerable — levels of radiation as it transits 
Neptune’s magnetosphere. Chapter 8 describes in more detail what was 
done in the encounter sequence design to deal with Neptune’s environmental 
hazards. Later discussion in this chapter will fill you in on more of the actual 
encounter events and scenes. 

Setting Up for the Encounter 

Skimming Voyager 2 over Neptune to get it close to Triton raises 
another concern that has received considerable attention over the past two 
years or so: navigation. There are lots of uncertainties at Neptune, such as 
its exact position and size: ditto for Triton. Plus, we never know exactly 
where Voyager 2 is at a given time, much less where it will be months in the 
future. (Actually, considering how far away this is happening, we know 
these values fairly well, but still not well enough to do a perfect job.) All of 
these uncertainties pile up to make navigating through Neptune’s domain 
a formidable task. 

For example, although the selected trajectory promises to give us a tour 
we’ll never forget, it’s still very sensitive to aiming-point changes. This is es- 
pecially true for the Triton science, because relatively small position errors 
at Neptune closest approach tend to get magnified by the gravitational 


bending effect, leading to errors nine times bigger by the time Voyager 2 
arrives at Triton. Arrival-time errors also get magnified. This means that 
good navigation — good shooting at our aiming space target — is a must for 
this encounter. 

How big is this spot we’re aiming for? Considering how far away it is 
from Earth, it is unbelievably small to most people. The skilled Voyager navi- 
gators will try to guide Voyager through an imaginary needle's eye about 
100 km (62 mi) wide, while the spacecraft is going a blistering 27 km/sec 
(nearly 61,000 mi/h) — and they expect to predict when this will happen to 
within one second! Threading this needle at Neptune closest approach — just 
above its cloudtops — will ensure with about 85 percent confidence that 
Voyager 2 passes through the Earth and Sun dual-occultation zone behind 
Triton 5 hours and 43 minutes later. (We would like to achieve a 90 to 95 
percent probability of success for this dual occultation, but 85 percent is 
about the best we can do, given the uncertainties.) 

So how does one become a good shooter? One way is to take several 
shots. For a number of reasons, Voyager 2 is nudged to the nominal aiming 
point over several years with a series of Trajectory Correction Maneuvers 
(TCMs), rather than one big maneuver just before Neptune. The general 
strategy has been to ease Voyager down to the desired aiming point from a 
safer one farther out from the planet, away from the hazard zones. As we 
learn more and more about the Neptunian system during Voyager’s ap- 
proach, these maneuvers become more refined and precise, homing in on the 
desired aiming point with more confidence. One such TCM was executed in 
early 1987, and another in late 1988. TCM B17C (described below) was just 
completed in April this year. Three more are planned during the encounter 
period as well. 

Striving for Perfection 

In Chapter 3, you were introduced to the concept of Computer Com- 
mand Subsystem (CCS) loads, the computer programs stored on board 
Voyager to control its activities. At any given time, one of these programs 
must be running, or the spacecraft will cruise along doing nothing. For the 
Neptune encounter, ten CCS loads have been de- 

TIME :-0 Days. 0 : 50:00 . 

1989/08/25 3:10 gmt (sct) signed to execute the desired sequence of events. One 

additional load is ready to support a contingency 
option also. The contents of each CCS load will be 
described later in this chapter, but now is a good time 
to introduce you to their names. 



Every Voyager-2 CCS load, except the contingency load, starts with the 
letter “B”; Voyager-1 loads start with “A”. The OB phase is composed of three 
loads: B901, B902, and B903. The FE phase has three as well: B921,B922, 
and B923. NE has only two, B951 and B952. B951 is the load that includes 
the closest approaches to Neptune and Triton, and is very complex compared 
to the others. The encounter ends with two loads in the PE phase, B971 and 
B972. The sole contingency load for the Neptune encounter is called R951, 
for reasons explained in Chapter 8; it backs up the high-value B951 load. To 
keep Voyager-1 cruise activities to a minimum during the Voyager-2 Nep- 
tune encounter period, a very long CCS load known as A818 will keep 
Voyager 1 going from just after the start of OB until more than two months 
after PE has ended. 

Completing these loads and getting them “on the shelf," as we say, was 
no easy task. In fact, it took about two years of preparatory work (1984- 
1985), and over three years of intense activity (1986-1989) by nearly all 
facets of the Voyager Project to do it, with assistance from many other 
individuals sprinkled around the globe. Even though they are on the shelf, 
these loads are not the best we can do, so even while you read this Guide, they 
are being updated. 

The encounter CCS load development process essentially got started in 
February 1984, when a three-day workshop was sponsored in Pasadena by 
the Voyager Project to establish a scientific framework within which to plan 
the Voyager Uranus and Neptune encounters. Regular meetings of the 
Voyager Science Working Groups (SWGs) in 1984 and 1985 refined the Pro- 
ject’s understanding of the Neptune system and the important scientific 
issues that could be addressed effectively by Voyager once it arrived. By the 
Uranus encounter, a preliminary version of the first month or so of the 
Neptune encounter (essentially the OB phase) had already been developed, 
but further work had to be put on the back burner to concentrate on Uranus. 

Following the highly successful Uranus encounter in early 1986, 
everyone got busy on the Neptune CCS loads; the work has been non-stop 
since then. Constructing high-fidelity timelines of the desired encounter 
sequence of events proceeded by encounter phase — not in chronological 
order, but saving the most difficult designs for last (OB, PE, FE, and NE). 

This design process consisted of five phases: Guidelines Development, 
Scoping, Integrated Timeline, Final Timeline, and Uplink Product. By the 
end of the Uplink Product phase, each CCS load is supposed to be mature 
enough to actually work on board Voyager, but perhaps not optimally. The 
last load to finish this phase was put on the shelf in April this year — over 
three years after the process got a serious start! 


But we want these computer programs to be perfect, remember? To 
approach this goal, each load is then taken “off the shelf’ (chronologically this 
time) and run through another step in this grueling process called the 
Updates phase. Each load is updated with the latest geometric and timing 
knowledge, and sometimes subjected to a few minor sequence modifications 
to correct errors discovered, to eke out a bit more science, or to perform 
something more efficiently or safely. The first load of the encounter (B901) 
started this phase in March this year, and was done just before it was sent 
to the spacecraft in early June. Each subsequent load follows this example. 

There is more to say about this Updates phase, because we’re not done 
yet! Now the real fun begins. 

Critical Late Activities 

The geometric and timing updates that each load receives before it is 
uplinked are called a Late Ephemeris Update, or LEU. To do one of these, 
updated estimates of Neptune system physical constants (body sizes, masses, 
locations, etc.) and the latest description of Voyager’s trajectory are fed into 
a fancy computer program that calculates all of the changes necessary in a 
given CCS load to re-center its observations and reset the various timing 
relationships between activities. Feed these updates and the “outdated” 
CCS load to the computer, and out comes a more nearly perfect load — just in 
time to send it to our distant recipient. This is good enough for most loads, 
but not all of them! There’s more . . . 

Imagine you are heading for Neptune, riding along with Voyager 2. For 
months and months, the planet just seems to hang there in front of you, 
growing in size, but not very fast. The view is almost boring, it’s so constant. 
Then, in the last month or so, you get closer and closer, and the detail starts 
popping out at you. Fairly suddenly, you note sizes and positional relation- 
ships with unprecedented clarity and accuracy. Pictures are taken that can 
be used to pin down the orbits of the planet and moons. You see things never 
seen before. In short, in just a few weeks, all of the uncertainties you had 
been struggling with for years begin to shrink down to a mere fraction of their 
prior values. The Neptunian system comes alive for the first time ever, and 

models transform into reality. 

TIME: -0 Days. 0:45:00 
1989/08/25 3:15 GMT (SCT) 

This situation is going to happen in late July and 
all through August, and a plan is needed to deal with 
all of these late improvements in our knowledge. For 
the CCS loads that orchestrate Voyager’s activities 
close to Neptune, special updates will be needed to 
factor in the most current data at the latest possible 



65916 KM 

time — so late, in fact, that these updates must be sent to Voyager after the 
CCS load has already started executing! Reliance on the LEU alone simply 
is not good enough, because the most sensitive observations would fail if 
attempted without the latest information, and critical maneuvers to fine- 
tune the aiming point could miss the planned target by more than the 
sequences could tolerate. 

Two special update techniques are invoked to make these critical 
updates. To update selected observations, a procedure similar to the LEU, 
called the Late Stored Update or LSU, is used. For putting some “English” 
on the TCMs, a TCM “tweak” is done. 

LSUs will be required for two loads: B951 and B971 . One is needed for 
B951 because it controls activities closest to the planet, where the effects of 
the various uncertainties are most evident. The B971 load demands an LSU 
because we won’t know — until it happens — exactly at what angle Voyager 2 
will be leaving Neptune after its gravity-assist slingshot over the northern 
polar region, and several high-value observations in B971 are sensitive to 
this angle. Predicting this outbound trajectory beforehand is beyond our 
capabilities, so we’ll tell Voyager what it is — once we can pin it down — with 
an LSU. 

The TCM tweak technique will be used in B903 for TCM B18, in B922 
for TCM B1 9, and in B923 for TCM B20. For TCM B1 9 and TCM B20, a tweak 
option even exists to make Voyager 2 do a last-minute trajectory “bailout” 
maneuver, to avoid any environmental hazard surprise discovered during 
the last few days before Neptune closest approach. 

Developing the sequence of events for all of these late updates — the 
Late Activity Timeline — required a considerable effort by most of the 
Voyager teams. The process started in early 1987 and, after a great deal of 
study and analysis, by mid-1988 a reasonable timeline for the LEUs, CCS 
load uplinks, LSUs, and TCM tweaks had been worked out. The busiest part 
of these “late activities," as they are called, is shown in Figure 6-4. Figure 
6-5 breaks down a typical late activities block into its component steps. 

For most of August, the late activities will be keeping many encounter 
teams busy around-the-clock. The Navigation Team will gather data from 
Voyager 2 to improve their mathematical models of the Neptunian system 
and Voyager’s orbit; the scientists will evaluate these new models and 
recommend changes to the various parameters that control their observa- 
tions; maneuver designers will request minor changes to the TCM designs; 
spacecraft and sequencing experts will push each CCS load through a series 
of computer programs to incorporate these changes and check for errors; 
operations people will grease the skids to make sure the updates are sent to 


Figure 6-4. Neptune Encounter Late Activity Timeline. 

ui l 
W o 


i A 

^ o 

w o 


B971 LELl[ 

B971 LSU^ 


B952 LEU[ 
B951 LSU ( 

uplink ni - 



B951 LEUj^ 
B923 UPLiNk|jjjj 

B923 LEU[ 


B922 LEU£ 



□ □ 

RS-0RT4 TCMB18^ 














I LSU [ | 









































n n. n ■ n , 

n i n i 

- 21 ° - 20 ° -19° 


q i — d j □ j cl rr i n i □ ■ 


n i n . n , n . n , . r~i 


-13 u -12 u -11 u 

.gD .gD . 7 D .gD . 5 D . 4 D . 3 D 



n, n , 





Figure 6-5. The Voyager encounter teams will work around-the-clock in a fast- 
paced effort to incorporate the latest knowledge about Neptune's system into 
the CCS loads , using a series of activity blocks like this one . 

the spacecraft on time; and mission management will be hosting meeting 
after meeting to approve the change requests and maintain coordination. 

If we pull off the late activities as planned — and we are confident we 
shall — Voyager 2 will execute CCS loads that are as perfect as its keepers on 
Earth can supply. 

Practice Helps to Make Perfect , Too 

After the preliminary Late Activities Timeline had been developed in 
mid-1988, a new phase in encounter preparations began: Test and Training. 
As the name implies, this activity is concerned with testing some of the clever 
ideas planned for Neptune (see Chapter 9) and conducting some rehearsals 
of the more complex periods of the encounter (including the LEUs and LSUs), 
so we don’t try everything “cold turkey." 

This work began essentially one year before the start of the encounter, 
and ended virtually at the start of B901, in June 1989. It, too, required a 
major effort by nearly all Voyager encounter teams. Refining all of the 
preliminary notions of the various encounter activities into final, precise 
descriptions that satisfy everyone — and then checking them all out — was by 
no means a simple job. 


The Test and Training phase was a mixed bag of activity, involving 
nearly all Project teams and the generation of lots of detailed plans and 
procedures for every team to follow during the various exercises. Since 
October 1987, a series of Operational Readiness Tests (ORTs) has been 
conducted to validate and calibrate the Earth-based communications net- 
work to be used for gathering the important radio science occultation data 
coming from Voyager 2 as it passes behind Neptune and Triton. These “RS- 
ORTs” continue well into the encounter. Special Capability Demonstration 
Tests (CDTs) were conducted on Voyager 1 (our “testbed”) and Voyager 2 in 
1988 and early 1989 to verify new spacecraft operating techniques and to 
gather some performance calibration data. One of these CDTs completed in 
April with Voyager 2 served a dual purpose: it validated the roll-turn TCM 
concept planned for use with TCM B20 (see Chapter 9), and in the process 
performed another TCM, called TCM B17C. 

As a final dress-rehearsal for the encounter, a high-fidelity Near En- 
counter Test (NET) was executed in May on board Voyager 2. It simulated 
the most complex chunk of the B951 sequence, with everyone on Earth 
playing their appropriate roles. Now, it’s time for the real thing. 

How the Bits Flow 

To maintain its busy schedule of activities, Voyager 2 requires periodic 
bursts of instructions from Earth. Various teams at JPL, in turn, demand 
near-daily infusions of Voyager engineering and science data and ground- 
computed navigation data to help them keep track of what is happening at 
its distant locale. In Chapter 3 you were introduced to the system that 
facilitates this flow of information: the Ground Data System (GDS). 

As the excitement of the encounter builds, more and more images and 
data plots will be displayed on TV monitors at JPL only minutes after the raw 
information required to display them arrives at Earth from Voyager 2. The 
casual observer may think that the spacecraft is beaming everything we see 
on the TV monitors directly to JPL, much like a cable TV satellite does when 
it transmits a newly released movie or popular sporting event to the local 
distribution station or even individual homes. But in the case of Voyager 2, 
this is simply not what happens; the digital data bits 
must first zip around a fair part of the globe via the 
GDS before they reach JPL and those eagerly await- 
ing their arrival. 

Figure 6-6 shows the telecommunications “high- 
way” for the first part of this journey — after the data 
travel over 4.4 billion km from Voyager to Earth, that 


TIME: -0 Days. 0:35:00 
1989/08/25 3:25 GMT (SCT) 


54921 KM 

Figure 6 - 6 . Land-based telephone lines , land-based microwave links , submarine 
cables and communications satellite links are all fair choices for constructing 
possible data paths with the GCF ) ultimately , availability , reliability , and cost 
dictate which route is best. 

is! From one or more antennas at the DSN sites (California, Australia, or 
Spain), the data bits (including navigation data) are routed to JPL by one of 
many possible routes arranged ahead of time by NASCOM telecommunica- 
tions network personnel based at NASA’s Goddard Spaceflight Center in 
Greenbelt, Maryland. The NASCOM people employ a world- wide switching 
and routing system called the Ground Communications Facility (GCF) to get 
their job done. 

For the Neptune encounter, two special cases for routing Voyager data 
arise. The signals from the Very Large Array in New Mexico are sent via 
satellite link to the Goldstone, California, DSN site, where they are com- 
bined (arrayed) with Goldstone’s signals; the product is then relayed to JPL 
via the GCF. At Usuda, Japan, the radio science occultation signals are 
recorded as they are received, and tapes of the recorded data are then sent 
directly to JPL for further combining with the comparable signals that 
arrived via the GCF from Australia. This combining takes place days after 
Neptune closest approach, since shippingtapes by conventional mail is much 
slower than electronic transfer of data. 


Data arriving at JPL via the GCF are first manipulated in the Mission 
Control and Computing Center (MCCC), in particular the third floor of 
Building 230 (see Figure 6-7). Note that commands to Voyager leave the 
MCCC, pass through the GCF, and get transmitted to the spacecraft by the 
appropriate DSN antenna. 

The MCCC is where the data are decoded and identified for future use 
by the Data Capture and Staging (DACS). After passing through the DACS, 
the data are sent to the Test and Telemetry Subsystem (TTS) for display, to 
the Data Records Subsystem (DRS) for archiving and storage, and to the 
Multimission Image Processing Subsystem (MIPS), where some of the initial 
image processing is done. From the MCCC, the data are routed in various 
specialized formats to several sites on Lab, as depicted in Figure 6-7. 
Processing of these data streams is done in real time and non-real 
time, depending on the data type and ultimate use. 

Science data of all types are sent to the second floor of Building 230, 
where many Voyager Principal Investigators and their staffs will be sta- 
tioned. Radio science data are routed to the third floor of Building 264, where 
a special program called RODAN deconvolves the signals into scientifically 
meaningful numbers. An array of science support workstations on the 
second floor of Building 301 (known as VNESSA, the Voyager Neptune En- 
counter Science Support Activity) and on the third floor of Building 264 
(known as VISA, the Voyager Imaging Science Activity) also receive their 
own data feeds, as do additional MIPS computers in Buildings 168 and 169. 
Optical navigation (OPNAV) images are processed on the second floor of 
Building 264 using dedicated computers and specialized software. And, last 
but not least, von Karman Auditorium is connected to most of the other sites 
to ensure that encounter surprises are displayed and routed to the thou- 
sands on Lab and millions off Lab who anxiously wait to see them. 

Neptune at Last! 

All of the discussion above was included in this chapter to get you 
oriented and sensitized to the enormous effort required to plan for this 
encounter. It hasn’t been easy. What we have to show for the past three and 
one-half years is ten nearly perfect CCS loads and an 
incredible amount of planning and preparation. The 
rev/ard for everyone’s hard work will be the show that 
Voyager 2 sends back from Neptune. 

With this appreciation, sit back now and relax. 
It’s time for the final stop on Voyager’s Grand Tour of 
the outer solar system . . . and here we go! 

TIME: -0 Days, 0:30:00 
1989/08/25 3:30 GMT (SCT) 



49620 KM 

Figure 6-7. Data bits can travel down a number of possible paths before they arrive 
at their final destinations. The MCCC at JPL serves as the central switchboard for 
most data arriving at the Lab. 


Observatory Phase (OB) 

The name of this phase describes its primary nature well: it is a 
repetitive two-month watch of the Neptunian system, which to some will 
seem almost monotonous. For most of those on Voyager just finishing the 
hectic Test and Training phase, the relatively slow-paced OB events will 
come as a refreshing break (although updates of subsequent loads will 
prevent much relaxation); the teams staffing the DSN sites are notable 
exceptions to this trend, however, since their workload actually increases at 
the start of this phase. Figure 6-8 highlights the last month of Uranus-to- 
Neptune cruise activities and all of those in OB. Note that OB starts with 
a fairly active load, which is followed by a relatively inactive load twice as 
long, and ends with a short, relatively active load. 

The first OB CCS load, B901, starts at 80 days, 21 hours, and 17.6 
minutes before Neptune closest approach (N-80d 21 h 17.6m, for short), and 
ends exactly 18 days later. (Refer to Table 6-2 for a listing of the start times 
and durations of all ten encounter loads.) Two hours into the load, the first 
of many executions of a link called VPZOOM gets underway. (Refer to the 
end of Chapter 2 for a summary of how these science links are named.) Each 
VPZOOM is designed to take five Imaging Science Subsystem (ISS) narrow- 
angle (NA) camera pictures of Neptune every one-fifth of a planetary 
rotation (one-fifth of a Neptunian “day”). By stringing together all of the 
VPZOOM frames taken during the same times of day, five movies will be 
made to highlight the visible atmospheric features marching across Nep- 
tune’s disk. Scientists can then use these movies to estimate the velocities 
and dynamics of the various cloud features and bands — some of which have 
been observed for several months now, though not very clearly. A link 
similar in emphasis to VPZOOM, called VPMOVIL, is especially tailored to 
bring out faint details in Neptune’s atmosphere. 

Shortly after VPZOOM starts, surveys of Neptune with the Ultravio- 
let Spectrometer (UVS) also begin, using the USSCAN, USMOSAIC, and 
UPAURORA links, among others. USSCAN looks for signatures of neutral 
hydrogen and excited ions in a one-dimensional scan across the entire 
Neptunian system (out to beyond Triton’s orbit), while USMOSAIC does the 

1989/08/25 3:35 GMT (SCT) 

TIME: -0 Days, 025:00 

same in a two-dimensional array. UPAURORA dwells 
on Neptune only, looking for signs of auroral dis- 
charges near the planet’s south pole. These broad 
surveys continue for all of OB. 

Bursts of Planetary Radio Astronomy (PRA) and 
Plasma Wave Subsystem (PWS) data are taken once 
per day throughout OB as well; the frequency of the 


44546 KM 


Figure 6-8. Voyager-2 Neptune encounter overview timeline for late Uranus-to- 
Neptune cruise and the Observatory phase . 


Table 6-2. Ten CCS loads will be used during the four-month Neptune encounter. 





Start Time 

Start Time 
(Enc. Relative) 





Jun 05, 



21 h 






Jun 23, 








Jul 28, 








Aug 06, 









Aug 1 3, 








Aug 19, 









Aug 24, 








Aug 26, 


+01 d 







Aug 29, 








Sep 11, 











Oct 02, 




6 weeks 

PRA signal in particular will likely be the best indicator of Neptune’s core 
rotation rate. 

Three days into B901, the last scheduled scan platform Torque Margin 
Test (TMT) is performed (refer to Chapter 9). This one-hour test measures 
friction levels in the azimuth (Az) and elevation (El) actuators. 

One day after the TMT, the spacecraft gyroscopes (gyros for short) are 
turned on to support about two and one-half weeks of calibration-related 
activities. First among these, at about N-73d 15h, is a calibration of the High 
Gain Antenna (HGA) signal pattern and a Sun sensor alignment check. This 
calibration also takes about one hour to complete; you may see it referred to 
as an ASCAL. 

A little over three days after the ASCAL, the spacecraft executes a 
tumbling-like attitude maneuver that to the uninitiated observer would 
certainly suggest Voyager 2 was out of control. This Cruise Science Maneu- 
ver, or CRSMVR, involves cartwheeling the spacecraft end-over-end four 
times in yaw (about the Y axis) and four times in roll (about the Z axis) to 
sweep the magnetometer (MAG) sensors on their long 
boom through the interplanetary magnetic field in 
order to determine where “zero” is, and also to meas- 
ure the local magnetic field generated by the space- 
craft itself. Voyager has been doing this calibration 
for years now, so it is not as tricky as it seems at first. 

TIME: -0 Days, 0:20:00 
1989/08/25 3:40 GMT (SCT) 


39823 KM 


Right after the CRSMVR, the gyros are turned off and Voyager initiates 
a routine that determines the orientation of the MAG boom itself — in 
bending and twisting — within two degrees. The remainder of B901 (another 
week) includes more of the repetitive observations already described. 

At roughly N-63d 20h, the transmission of CCS load B902 begins 
arriving at the spacecraft, having been sent some 3h 58m earlier from Earth. 
A little less than one day later, B901 clocks out and lets B902 take control of 
Voyager’s activities. 

B902 continues with the systems-level observations started in B901, 
and also includes calibrations of the UVS and Photopolarimetry Subsystem 
(PPS) instruments. Between N-60d 2h and N-59d 15h, these instruments 
are pointed at various stars to get baseline data for later comparison to 
similar data to be taken during the Neptune, Triton, and ring-arc system 
occultations in B951 and post-Neptune calibrations in PE. 

Between N-58d and N-57d, RS-ORT 3.5 is conducted; it is a scaled- 
down version of RS-ORT 3, performed in conjunction with the NET in May. 
This ORT exercises the Neptune-Earth occultation sequence in B951 with 
the extensive DSN arrays on Earth, including the Usuda site in Japan, a 
newcomer for this encounter. For the first time in the encounter, the S-band 
transmitter will be turned on during this test, but only for a few hours. 

If there is to be such a thing as a lull during the encounter, the month 
following RS-ORT 3.5 has to be it. Between about N-57d and N-28d, our 
untiring explorer executes — what else? — a continuous stream of Neptune 
system scans. The only significant exceptions to this routine are the turn- 
on and checkout of the Infrared Interferometer Spectrometer and Radiome- 
ter Subsystem (IRIS) at N-38d and another PPS/UVS star calibration at N- 
31 d, just before the end of B902. Weekly Very Large Array (VLA) passes are 
scheduled into the DSN coverage as well. (The VLA is also supporting 
Voyager for the first time this encounter.) 

B903 starts at N-28d with a command to turn the Voyager gyros on; 
they remain on until almost the end of the encounter. After a CCS timing 
test three days into the load, the tweak for the first encounter trajectory 
maneuver, TCM B18, is uplinked from Earth. Since a bit more data are 
returned during this last week of OB, a little extra DSN coverage is planned, 
as one can see in Figure 6-8. 

It hasn’t been mentioned until now, but all during OB (and since TCM 
B17C in April), Voyager has been beaming back a fairly steady stream of 
optical navigation images (OPNAVs) of Neptune, Triton, and Nereid to help 
navigators on Earth determine where everything is in the Neptunian 
system, and where Voyager 2 is heading. Anything new learned along the 


way is factored into the TCM B1 8 tweak. By N-24d 3h, the tweak parameters 
are loaded in the CCS in time for TCM start at N-23d 16h. 

TCM B18 is designed to remove all known errors — aiming-point posi- 
tion errors and arrival-time errors — from the Voyager trajectory. At just shy 
of a month from Neptune, we still don’t know exactly how far off we are from 
the desired aiming point, but we know a lot more than we did at TCM B17C 
last April! 

To ensure the total electrical power usage onboard Voyager does not 
exceed allowable limits during the TCM, the radio transmitters are reconfig- 
ured from X-band high power/S-band off (X-HI/S-OFF) to X-band low power/ 
S-band low power (X-LO/S-LO) just before the TCM starts. Once the 
maneuver has executed, the system goes to X-HI/S-LO, and stays that way 
until the next ORT in early FE. The X-LO/S-LO configuration is used during 
TCM B19 as well. 

The TCM involves significant power state changes on the spacecraft as 
well as attitude changes; both of these influences alter Voyager’s thermal 
state significantly, and thus induce a relatively long command moratorium 
of three days (refer to Chapter 4). While everyone on Earth is waiting for the 
command moratorium to end, Voyager executes a UVS Sun calibration at N- 
23d lOh, checks the IRIS health, and resumes its Neptune system scans. For 
the first time in the encounter, a concerted effort to detect very small 
satellites begins at N-21 d 1 h, using the VSATSRCH link. Movies of the ring- 
arc region are radioed back on a daily basis as well, in hopes of seeing some 
evidence for material there at this early date. 

Commandability returns near the end of OB at about N-20d, just in 
time to uplink the first FE load, B921. 

Far-Encounter Phase (FE) 

At the start of the encounter, Neptune was over 11 weeks away, and its 
disk and ring-arc system, combined, only spanned one-sixth of the NA 
camera field of view. Now, at N-18d 19h, Neptune’s disk alone captures 
about one-quarter of this view, and things look much more interesting. The 
end of this phase leaves Voyager 2 as on-target as it will ever be, and only 

TIME: -0 Days. 0:15:00 
1989/08/25 3:45 GMT (SCT) 

twelve hours from Neptune closest approach. 

As you can see in Figure 6-9, the first of the three 
FE CCS loads is the longest, but still only a half-day 
longer than a week; the other two clock out in less 
than a week. Needless to say, these shorter durations 
are indicative of a higher level of activity than we saw 
in OB. More activity means more things for Voyager 



35638 KM 

UJ ^ 





CJ 2 
w CO 


30 31 1 

2 3 4 5 6 7 6 9 10 1 

1 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 1 

2 3 4 5 6 7 





-11 -4 -0 ♦3 






T ▼ 


< o 

<oi § < 

+ i < o 
o o o 

QL 3 

8 § 












▲ ▲ 

3 9: | 

o ^ I 

<U uj 
m < x 
CO »- CL 

^ DC g 

p o o 

p§ < m 

I- UJ 




- NO BML- 


















30 31 1 2 3 4 


7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 3' 

11 2 3 4 5 6 7 

Figure 6-9. Voyager-2 Neptune encounter overview timeline for Far -Encounter and 
Near-Encounter phases. 


to do, and more CCS words required to do them. In active periods, the CCS 
just can’t control things for long before its memory space is used up. 

More activity also means more telemetry data, so the DSN kicks into 
high gear at the start of B921 with daily VLA passes in addition to the 
amount of arraying used in late OB. The FE phase also marks the start of 
the busiest series of late activities, depicted in Figure 6-4. 

B921 starts with an intensive series of system scans, much like OB, 
only more. The UPAURORA links are executed four or more times per day, 
as are a variation of VPMOVIL, called VPNAMOVI, which focuses on the 
large-scale features in Neptune’s atmosphere. The VSATSRCH links and 
probing of the ring-arc s} r stem continues, and more links train the sensors 
on Triton as well. The PRA and PWS data bursts are beamed back to Earth 
twice per day. 

Starting less than one day into B921 , the last full-scale dress rehearsal 
for the encounter, RS-ORT 4, is conducted. The test involves lots of Voyager 
and DSN people worldwide, and lasts about ten hours. Anticipation mounts. 

Uplinking the parameters for either a backup TCM B18 or contingency 
TMT (should either be needed; see Chapter 8) is scheduled for N-17d 3h, just 
after the command moratorium caused by RS-ORT 4 ends, and eight hours 
in advance of the planned start time for the contingency sequence. 

At N-16d 9h, the Navigation Team takes all optical and radio data 
returned from Voyager up to this time and starts solving for the latest 
Voyager-2 trajectory. With this solution in hand, the detailed design for the 
next planned maneuver, TCM B19, proceeds. (The data cutoff for the TCM 
B19 tweak occurs at N-12d; the maneuver itself executes at N-9d 18h.) 

At N-13d 13h, another IRIS health check is performed in preparation 
for the high-value RPDISK observation scheduled in B922. Through the end 
of the B921 load, most of the pre-encounter instrument and spacecraft cali- 
brations are finished, and all of Voyager’s sensors are kept busy observing 
Neptune, Triton, Nereid, and the ring-arc system. Included in the returned 
data stream are various optical and radio data-types needed to support the 
upcoming all-important TCMs, B19 and B20. 

And the view is getting better: at N-12d, the ring-arc system fills the 

entire NA field of view. 

TIME: -0 Days, 0:10:00 
1989/08/25 3:50 GMT (SCT) 

In the six-day B922 load, which starts at N-lld 
5h, more and more details appear for the first time. 
Faced with so many science opportunities, more CCS 
memory would be welcome. To accommodate this 
need, the sixth version of the Backup Mission Load 


C - 7- 


32257 KM 

(BML 6) is removed from the CCS late in B921 to free up much-needed 
memory space. 

At this time, the Navigation Team is working quickly to solve for the 
TCM B1 9 tweak values. By now, the uncertainties that everyone fussed over 
for so many years are dropping precipitously, thanks to Voyager. Neptune’s 
position is now known three times more accurately than just two weeks 
earlier; Triton three to six times better, depending on the component. Time 
of arrival is one-third better. The value of Triton’s radius is no longer a 
mystery. And by this time, the mass of Neptune is known three times better 
than at the start of FE, because the subtle tug of the planet’s gravity can 
already be detected in Voyager’s radio data. Our explorer is accelerating . . . 

The TCM B1 9 tweak is uplinked to Voyager 2 at N-l Od 9h, and 1 5 hours 
later the maneuver executes. This TCM is designed to remove all estimated 
trajectory position errors, but only some of the estimated time-of-arrival 
error. The movable blocks in B951 (more on these later) take care of the 
residual timing offset. The Navigation Team estimates that there is about 
a 25 percent chance that TCM B19 will not be needed, given that TCM B20 
remains to remove some position errors. 

Immediately following TCM B19, Voyager performs a turn about its 
roll axis (Z axis) to acquire a different lock star for its star tracker. This 
change from Achernar to Canopus places the onboard fields and particles 
instruments in a better position for measuring magnetospheric properties 
during the days before Neptune closest approach. 

The RPDISK observation in B922 from N-9d to N-7d is the highest- 
value science observation in the FE phase; the matching outbound observa- 
tion in PE is equally important. Why? Because one of the most meaningful 
measurements to get for planets (especially the gaseous ones) is the heat 
balance — the difference between how much heat a planet receives from the 
Sun and how much it gives off. Knowing this, we can deduce things about 
the body’s interior, and unlock some of the secrets hidden by its clouds. At 
around N-7d 12h, Neptune’s disk should just barely fill the IRIS field of view; 
this is the optimum time to make the heat balance measurement. (Neptune 
fills a bit over one-half of the NA camera view at this time; Triton is still very 

One day after the RPDISK observation, at N-6d, the uplink for the final 
FE load starts. About 18 hours later, this load, B923, takes control of 
Voyager 2. Neptune looms ahead, almost filling the entire NA camera field 
of view. 

B923 is sequenced with specialized observations of Neptune and its 
atmospheric features, its ring-arc system, Triton, and Nereid. All eleven 


Voyager investigations are employed. There are even some images reserved 
for moons we didn’t know existed at the start of the encounter — but 
suspected they would be found. Fields and particles instruments continue 
their search for Neptune’s magnetosphere. The unambiguous sign of 
Voyager’s entry into this strange domain of whistlers, chirpers, and electro- 
magnetic static and hissing is the magnetopause crossing, estimated to occur 
between about 27 and 9 hours before Neptune closest approach. 

In parallel with Voyager’s heightened activity, the activities in and 
around JPL quicken, as the various Voyager teams prepare for Neptune and 
Triton closest approach, and as the rest of the world takes a serious and 
public interest in what is happening far away at Neptune. 

The late activities schedule really picks up during B923. The TCM B20 
tweak, B951 LEU, B951 LSU, B951 uplink, B952 LEU, and B971 LEU keep 
most of the support teams busy night and day. The Navigation Team works 
especially hard to pin down all of the values needed to support the critical 
TCM B20 tweak design and B951 LEU and LSU updates. 

The final aiming point for the encounter is fixed when TCM B20 starts 
executing at N-3d 16h 51m. This special roll-turn TCM is designed to adjust 
Voyager’s position such that the probability of it hitting the desired aiming 
point in Triton’s dual-occultation zone is maximized. After TCM B20, no 
further TCMs are planned — ever — for Voyager 2. Its fate is thus transferred 
to the final slingshot over Neptune’s northern polar region and the meager 
forces it may encounter in interplanetary and interstellar space. 

In less than five days from its start, B923 clocks out and hands over 
Voyager operations to B951. FE is over, and Voyager is ready to buzz 
Neptune and make its close pass by Triton. 

Near-Encounter Phase (NE) 

The complex B951 load lasts only two days and five hours (see Figure 
6-9), yet contains most of the high-value science we expect Voyager 2 to 
gather during the entire four-month encounter. It stands in a class by itself. 

With so much happening in this load and its companion, B952, these 
highlights will in the truest sense be highlights, because it would require an 
entire lengthy chapter to describe all of the activities 
in detail. Briefly, then, here is the NE phase. 

We pick up Voyager at N-12h 17m 36s, speeding 
towards its nominal aiming point at about 61,000 
km/h (37,700 mi/h), only 1 percent faster than its 
steady-state Uranus-to-Neptune cruise value. Nep- 
tune is still tugging, but not very hard . . . yet. 

TIME: -0 Days, 0:05:00 
1989/08/25 3:55 GMT (SCT) 


30016 KM 


By now, it is clear where Voyager is heading: Neptune completely fills 
the wide-angle (WA) camera field of view, which looks at fifty times more 
viewing area than the NA camera. Even Triton, for so long just a few pixels 
across even in the NA camera, spans half-way across the NA view. 

B951 starts with the X-band signal (during the XSGRAV link) con- 
trolled by a precise tone transmitted by the Canberra 70-m antenna. The 
DSN stations near Madrid then carefully listen to the return signal. With 
Neptune tugging on Voyager, there will be a measurable Doppler shift, 
which can then be used to deduce the strength of Neptune’s gravity. After 
XSGRAV gets started, some great full-disk WA images of Neptune are taken 
using the VPWA link. While the first set of these images is being shuttered, 
the fields and particles detectors kick into high gear with high-rate samples 
of the flow directions of magnetospheric charged particles every six minutes. 
Then the cameras are slewed over to take the best picture we’ll get of the 
small moon Nereid (via VNBEST), which will only fill about 20 pixels or so 
in the NA frame; Neptune was this size in January 1989. Next, a slew to 
Triton for a full-disk NA shot. 

It’s two hours into the load, and time for some classic Voyager science! 
Follow along with the visual view of what’s happening in Figure 6-10 as we 
accompany Voyager 2 on its trek. This timeline shows the order of, primar- 
ily, the imaging links as Voyager executes them; the longer timeline in the 
Guide’s “Hip Pocket” shows the order as received on Earth, including much 
of the late FE phase. Why the difference? Voyager can’t send us everything 
in real time — some observations must be recorded first, and then played 
back to us anxious Earthlings later. 

From N-lOh to N-8h, the IRIS is trained on a spot in Neptune’s 
atmosphere at -40.5 degrees south latitude, which is the latitude Voyager’s 
radio signal will pass through as the spacecraft pops out from behind the 
planet at the end of its Neptune Earth-occultation experiment, 55 minutes 
after Neptune closest approach. Using the data collected from this observa- 
tion (RPOCCPT), scientists can later determine the helium abundance at 
this occultation egress point, as it is called. These IRIS data provide pieces 
of the puzzle needed to determine the atmospheric structure and composi- 
tion there. 

After some ISS, IRIS, and PPS observations of Neptune’s sunlit limb 
(edge), Voyager trains its cameras on the expansive ring-arc region for a 
while. Between N-7h 17m and N-6h 22m, two executions of the retargetable 
ring-arc link VRRET1 are completed, employing for the first time the clever 
Nodding Image Motion Compensation (NIMC) technique described in Chapter 
9 to freeze the motion of selected clumps of orbiting ring-arc material. (The 


TIME: 0 Days, 0:00:00 
1989/08/25 4:00 GMT (SCT) 



29224 KM 




2 VTC 







r in I 

1 — 










A / 17 — ' 

i 6 

/ T V +-~_L_I - / 

i j\rr5 


\ l 


o \i — r r / / 










-2h NEPTUNE + 2h +4h +6h 




Figure 6-10. Data taken by Voyager 2 near Neptune closest approach. 


table accompanying Figure 6-10 summarizes the dramatic benefits of the 
smear-reducing techniques used in B951.) A long exposure of the region 
above Neptune’s north pole is made during this period as well in an effort to 
detect a hypothesized polar ring. 

By N-5h 18m, a scan of Neptune’s bright limb with the PPS and UVS 
is completed. Then for almost two more hours, it’s back to the rings. Between 
N-4h 55m and N-3h 3m, Voyager conducts two similar observations. In 
PRSIGSGR and URSIGSGR, the sensitive detectors in the PPS and UVS 
instruments gaze at the star Sigma Sagittarii as it drifts behind the right- 
hand half of the ring- arc system as a result of Voyager’s motion. This stellar 
occultation should reap a great harvest of detailed ring-arc region structural 
and orbital data. (This star helped us, in a sense, to point our finger at 
Voyager 2, remember?) 

While the bright limb scans and stellar occultations above are execut- 
ing, the bit stream containing the all-important B951 LSU parameters 
arrives and is loaded, having travelled from Madrid, Spain to the awaiting 
CCS in precisely 4 hours, 5 minutes and 57 seconds. Some extremely critical 
numbers reside in this LSU. For example, all Voyager science observations 
between about N-3.5h and N+9h are sequenced in what is called a movable 
block — three separate ones, actually. The first, the Neptune Movable Block 
(NMB), holds all activities around Neptune closest approach from N-3h 20m 
to N+lh 46m; another, the Triton Movable Block (TMB), contains the 
observations around Triton closest approach from N+lh 50m to N+8h 38m; 
the third, the Vernier Movable Block (VMB), encompasses the critical 
sequence for controlling the Neptune radio science occultation from N-5m to 
N+56m, and overlays the NMB. 

One thing the B951 LSU does is tell Voyager’s CCS units how much to 
shift these movable blocks in time. By allowing the entire block of activities 
in each block to shift, timing errors can be removed from the whole set in one 
simple step, instead of changing individual timing parameters in each 
observation. (There are so many observations during this busy period that 
using the piece-by-piece method would quickly use all available CCS words!) 

Shifts in multiples of 48 seconds are possible for the NMB and TMB; for 

TIME: 0 Days. 0:05:00 
1989/08/25 4:05 GMT (SCT) 

the VMB, a special technique is used that allows 
shifts in vernier multiples of only one second, inde- 
pendent of how much the NMB is shifted. The nature 
of the VMB is what forces the Navigation Team to 
estimate the time of closest approach to within one 
second. For everything except the critical radio sci- 
ence occultation, 48 seconds is good enough. 



30016 KM 

The other parameters in the B951 LSU control scan-platform pointing 
values to several high-value targets, maneuver rates for the radio science 
occultation maneuver, and rates for a critical Triton Image Motion Compen- 
sation (IMC) maneuver. 

So, with its timing corrected by the NMB shift, Voyager 2 continues. By 
N-3h, another NIMC-controlled VRRET1 is done. Next, the last great image 
of Triton before Neptune closest approach is taken: VTLON, again using 
NIMC. Triton subsequently gets eclipsed by Neptune’s southern limb, and 
won’t be visible again until the spacecraft arcs over Neptune’s pole. The scan 
platform shifts back to Neptune for some ISS, IRIS, and PPS photometry 
measurements. Neptune and Triton are so far apart now from Voyager’s 
point of view that it requires a medium-rate scan platform slew to go between 
them, just to save valuable time. The Low-Energy Charged Particle (LECP) 
instrument switches into a higher-energy sampling mode as Voyager 2 
penetrates the deepest part of Neptune’s magnetic field and radiation belts. 
The other fields and particles instruments also make their contributions to 
the flood of data. 

If you think things are busy now, think again . . . the next eight hours 
will really put Voyager 2 to the test! 

At N-lh 41m, a medium-rate, elevation-only slew points the sensitive 
optics of the scan instruments away from Neptune — towards deep space — 
to protect them for the impending ring-plane crossing. Then, one hour from 
its aiming point, Voyager configures its radio transmitter for the ring-arc 
system and Neptune occultations, calibrates its antenna, and gathers 
baseline pre-occultation data until N-20m. 

For about ten minutes centered around N-56m (plus or minus a few 
minutes — Voyager will be the first to know), the spacecraft crosses the ring 
plane just outside the suspected ring-arc region. The PWS instrument 
should pick up the sounds of microscopic (harmless) ring particles vaporizing 
as they hit the spacecraft. Voyager pops up and over this plane (as viewed 
from Earth) at 76,000 km/h (47,100 mi/h); it has gained 4200 km/h (2600 
mi/h) in the last 30 minutes alone! Neptune is tugging harder . . . 

Immediately after the expected ring-plane crossing, Voyager performs 
a 61 -degree roll from Canopus to orient the fields and particles instruments 
for measurements of the charged particles that should be raining into 
Neptune’s north pole along the magnetic field lines, perhaps causing auroral 
activity (“northern lights”). At the end of this roll, the spacecraft remains in 
All-Axes Inertial (AAI) mode, with its attitude controlled not by some outside 
source like the Sun or a star, but by the onboard gyroscopes. 


By N-30m, Voyager 2 reaches 85,000 km/h (52,800 mi/h), and the 
gravitational acceleration effects become noticeable: it is virtually getting 
pulled in by Neptune, and its trajectory is bending. Tiny Voyager must feel 
like the planet has grasped it firmly, and is now flinging it as hard as it can 
during the spacecraft's close dive over the northern polar regions. 

With pure X- and S-band tones emanating from Voyager, the radio 
science ring occultation (XBOCC) begins at about N-8m, although the fringes 
of Neptune’s ionosphere may affect these signals as early as N-20m. Back 
on Earth, the spacecraft rises above the horizon over the DSN sites in 
Australia. A bit more than four hours later, the signals from our remote 
beacon will land in the arrayed dishes, distorted in meaningful ways by their 
passage through the ionosphere, ring-arc system, and atmosphere. 

It’s N-5m. The duration of each of Voyager’s thruster pulses is 
increased from four-thousandths of a second to ten-thousandths, just in case 
Neptune’s atmosphere applies some unexpected drag on the vehicle, and also 
to provide quicker response to maneuver commands needed for the occulta- 
tion experiment. This special provision will remain in place for the next 
hour. The shift of the VMB precisely controls the timing for all occultation 
activities during this interval — V oyager’s finest hour at Neptune. Since the 
telemetry stream was turned off an hour earlier to concentrate power in the 
pure radio signal, all spacecraft telemetry is routed to the tape recorder for 
later playback. 

Our explorer’s speed relative to Neptune peaks at an impressive 98,350 
km/h (60,980 mi/h) as it silently and effortlessly sails through its aiming 
point — right on target — a mere 4400 km (2730 mi) above Neptune’s sensible 
atmosphere, and only 4850 km (3000 mi) above the methane cloudtops 
below. This is by far the closest Voyager 2 has been to any body since it left 
Earth twelve years ago. As it arcs over 77 degrees north latitude, it starts 
to slow down, and begins its permanent journey down and out of the ecliptic 
plane, thanking the gravity-assist effect for the ride. 

As the craft sinks behind the dark side of the planet, Neptune’s sunrise 
terminator passes beneath, and within about six minutes after closest 
approach, Voyager watches with a special UVS Sun-viewing port as the 
distant Sun disappears into the ever-thickening 
atmosphere. The Neptune occultation has begun. 

With its pure-tone transmissions still turned 
on — and while completely out of view from the Earth — 
the automated spacecraft performs an amazing series 
of 24 maneuvers, controlled to a large degree by the 
numbers that were stored onboard with the arrival of 

TIME: 0 Days. 0:10:00 
1989/08/25 4:10 GMT (SCT) 


32257 KM 


the B951 LSU. This string of maneuvers, collectively known as the “limbtrack” 
maneuver, precisely points the boresight of the HGA to Neptune’s limb, 
starting with the ingress point in Neptune’s northern hemisphere, then 
around the left limb (as viewed from Earth), and ending with the egress point 
at -40.5 degrees south. The limbtrack maneuver takes about 48 minutes to 
complete, and took an enormous amount of work to design. As Chapter 5 
explained, the radio signals are bent as they pass through Neptune’s 
atmosphere. The limbtrack maneuver controls the pointing of the HGA to 
ensure that these signals are bent so they hit the Earth and, thus, the DSN 
arrays, Parkes and Usuda included. We will learn a great deal about 
Neptune’s atmosphere, size, and shape from this experiment. 

Incidentally, while Voyager 2 is orchestrating its limbtrack maneuver, 
it is also collecting fields and particles data, collecting IRIS and UVS data 
from Neptune’s polar region, and also managing to take a series of three WA 
photos of the ring-arc system — this time from the other side, in forward- 
scattered sunlight. The last of these observations, VRHIPHAS, employs a 
new smear reduction ploy called Maneuverless IMC, or MIMC for short. 
Instead of moving the entire spacecraft smoothly to track the target, only the 
jerkier scan platform motion is used. 

As the spacecraft emerges from behind Neptune at N+55m 8s — again 
watching with the UVS — it sees the Earth first, followed by the Sun 49 
seconds later. It continues to point its HGA at Earth for the outbound 
XROCC, and takes a nearly edge-on shot of the ring-arc system (VRXING2) 
just before its descending ring-plane crossing at N+1.5h. (VRXING2 is a 
three-WA-image MIMC observation also.) Then, as a thin bright crescent 
begins to show in Neptune’s southern hemisphere, Voyager takes a parting 
shot — for the next eight hours, anyway — of Neptune’s limb. One of these 
crescent observations is the VPHAZE link, which employs Voyager’s classi- 
cal IMC for the first time during the Neptune encounter. 

With this phase of its mission done, the spacecraft focuses its attention 
on Triton, although its high-paced routine of fields and particles data-taking 
continues unabated. Back at Earth, everyone is still in the final stage of 
preparations for receipt of the first ring-plane-crossing data and the occul- 
tation signals! And the late activities continue, with the B952 LEU, B952 
uplink, and B971 LEU keeping everyone as busy as ever. 

A roll is completed to the Alkaid lock star at about N+2h, primarily to 
orient the charged-particle instruments for magnetospheric measurements 
between Neptune and Triton while, at the same time, preserving good 
viewing of Triton for the long-awaited upcoming observations. 


For the next three hours, Voyager soaks in its unique view of Triton 
with its IRIS, PPS, UVS, and ISS instruments. By now, the mysterious moon 
has grown to be twice the size of the NA view, and is still mostly sunlit. 
Figure 6-10 shows the three high-value observations from this period: 
VTCOLOR, VTMAP, and VTERM. All have been designed to bring out 
selected information from Triton’s surface; all use some kind of smear- 
reduction technique, and thus promise to be among the sharpest set of 
pictures Voyager 2 has ever returned. (Recall that the NIMC and IMC 
parameters were sent up with the B951 LSU, and that the timing for this 
chunk of science is controlled by the TMB shift.) 

By this time, Triton’s small gravitational tug can be felt by Voyager, 
allowing the scientists on Earth to measure the effects by changes in the 
radio signal. Speaking of those on Earth . . . many dozens are busy now 
monitoring the incoming Neptune occultation data; millions more are 
enjoying Voyager’s show. 

Triton closest approach occurs at N+5h 14m. Off in the distance, a star 
known as Beta Canis Majoris is about to get occulted by Triton. Voyager 
trains its PPS and UVS sensors on the star for about 20 minutes, watching 
its brightness change as it passes first through Triton’s wispy atmosphere, 
then behind the moon, and back out again. 

Then quickly, a medium-rate slew positions the UVS Sun port to the 
Sun, and the spacecraft configures its radio science equipment for another 
brief Earth and Sun occultation period. For nearly forty minutes, Voyager 
holds its attitude steady as it watches the two orbs of light, which wink out 
behind Triton from about N+5h 43m to N+5h 47m, recording all of its data 
as it goes. 

About 17 minutes after the Triton occultations, the spacecraft rolls 
back to where it started seven hours earlier, on Canopus lock, to permit 
unobscured viewing back towards Triton and Neptune. VTCRSCNT will 
show a thin bright sliver of sunlight smiling from the limb of an otherwise 
dark face of the moon. Next comes a two-hour series of IRIS, PPS, and UVS 
observations of Triton’s disk and atmosphere. 

It’s now N+9h, we’re at the end of the TMB. More importantly, the CCS 
is nearly out of words for sequence control! It’s 
amazing that Voyager just did so much in B951 with 
so little onboard memory (some 2200 18-bit words, or 
about 5 kilobytes), but thanks to the ingenuity of 
many Earthlings, it did. 

For 17 more hours, in conjunction with contin- 
ued frequent sampling of fields and particles data as 

TIME: 0 Days, 0:15:00 
1989/08/25 4:15 GMT (SCT) 


35638 KM 


Voyager passes through Neptune's magnetotail, the scan platform gazes 
back at the Neptunian system, taking in more observations with its various 

At N+l 3h, Voyager 2 sinks below the horizon in Australia. The Madrid 
DSN site steps in to continue our watch on Voyager. 

By N+ld, collection of fields and particles data slows down considera- 
bly, and Voyager takes a breather, its tape recorder nearly full. It is already 
over 1.5 million km (930,000 mi) from Neptune and has slowed to 61,200 
km/h (38,000 mi/h), only sixty percent of the speed it had just a day earlier, 
and only five percent faster than the speed of its eventual solar system 

Between N+21h and N+ld 16h, the IRIS and UVS scan from north to 
south across Neptune's disk in a repetitive sequence, having too few CCS 
words to do much more. At N+l d 1 6h 42m, the B951 load clocks out and B952 
carries on with the encounter, for exactly three more days. 

One of the big priorities in B952 is to unload the high-value data stored 
in Voyager's tape recorder and send it to Earth. This is achieved with a series 
of long playbacks scheduled over most of the load. In fact, as B971 (the first 
PE load) is being uplinked, the final playback is still in progress. The 
nominal plan is to have two playbacks of all high-value science completed by 

The long figure in the Guide's Hip Pocket shows two mosaics of the ring- 
arc system that come down from Voyager 2 in B952, VRMOS2 and 
VRNGARCS. In forward-scattered sunlight, these views may turn out to be 
quite revealing, as similar images were at the other encounters on the Grand 
Tour. A ring-arc movie much like those “filmed” during Neptune approach 
is also made during the last day of B952. 

Various IRIS, PPS, and UVS maps and scans are completed during this 
load as well. About midway through, VTAURZAP checks for visual evidence 
of aurorae or lightning at Triton. And for about 1 1 hours starting at N+3d 6h, 
the high-value UPCORONA link searches for UV emissions around Nep- 
tune's disk, which helps in the determination of the composition of escaping 

A change in lock stars occurs two days into this load. A brief 1 5-minute 
dwell on the star Spica allows the LECP instrument to get a better sample 
of the charged-particle flows in the downstream solar wind and their inter- 
action with particles in Neptune's nearby magnetic tail. About this time, 
Voyager 2 should be leaving Neptune's bubble-like magnetosphere and the 
surrounding bowshock, heading about 40 to 45 degrees south of the ecliptic 


By the end of the exciting NE phase, Voyager 2 will have already 
rewritten our understanding of Neptune and its neighborhood. Neptune's 
disk will again just about fill the NA camera view, but with a crescent this 
time, not a sunlit disk. Triton and Nereid — and probably a host of newly 
discovered objects — will appear as specks again. 

But the show isn’t quite over yet, because there’s another high-value 
science observation to get, and lots of post-encounter cleanup work for 
Voyager to do. 

Post-Encounter Phase (PE) 

PE lasts a bit over one month. The B971 load is not quite two weeks 
long, while B972 is about three, as Figure 6-11 shows. 

With the exception of a few Triton observations made by the UVS and 
ISS, all scan platform observations in this phase concentrate on Neptune 
and its ring-arc region. The highest-value observation in PE is the outbound 
RPDISK heat balance measurement, which starts just shy of a week after 
Neptune closest approach. 

Proper pointing of the scan platform is essential for a successful 
RPDISK. If the actual amount of gravity-assisted trajectory bending 
Voyager gets from Neptune differs sufficiently from the best pre-encounter 
estimates , N eptune may not be centered in the IRIS field of view a week later , 
after the spacecraft has had a chance to speed millions of kilometers away, 
slightly off its anticipated course. To accommodate this uncertainty, the 
outbound RPDISK design employs a nine-position mosaic centered around 
the estimated position of Neptune’s disk, rather than a steady gaze like we 
saw inbound. 

But even this feature may not ensure success, even with the B971 LEU 
factored into the equation, since the data cutoff for this LEU had to be before 
Neptune closest approach! Enter the B971 LSU. 

The data cutoff for the B971 LSU occurs at roughly the same time B971 
starts — well after a good estimate of Voyager’s outbound trajectory is 
available. Once the LSU reaches the CCS, the RPDISK observation is as 
good as done; the various teams that worked so long and hard for the previous 
three weeks on the late activities effort can move on to 
other encounter activities. 

By midway through B971, about the same time 
the LSU reaches the spacecraft, Neptune and its ring 
system combined will fit within the NA field of view. 
As is evident in Figure 6-11 , what’s left of the encoun- 
ter involves primarily calibrations, starting late in 


TIME: 0 Days, 020:00 
1989/08/25 4:20 GMT (SCT) 

UJ ^ 






o ^ 
</> o 




11 18 









+17 +24 









5 o 

▼ ▼ 

5 I 

5 < 

cc w 
< < 
H tc 

£ i 

0. o 

▼ T 











< b 
z < 
z £ 

m O 

z 1 

m m 

t- _i 
















. 1 . 





II* II.. 

iL fc day 


Figure 6-11. Voyager-2 Neptune encounter overview timeline for the Post-Encounter 
phase and early post-Neptune cruise. 


B971, and continuing throughout B972. These calibrations supply post- 
encounter data points to compare with those taken in OB and FE. They are 
very critical to the future encounter data analysis efforts, so great pains will 
be taken to get them all. 

With the uplink of B972, protection against failure of the remaining 
Voyager-2 receiver resumes via a small portion of CCS memory called the 
Voyager Interstellar Mission (VIM) Protection Sequence. This sequence 
provides limited BML-like protection, though much less than that designed 
into the pre-Neptune BMLs. 

Now and then in B972, Voyager 2 will snap some parting shots of 
Neptune, perform a scan with the UVS or PPS, and take a sample of the fields 
and particles environment. About a month after its spectacular passes by 
Neptune, the ring-arcs, Triton, and whatever else it finds there, Voyager's 
gyros will be turned off. Approximately a week later, the DSN coverage drops 
to a bare-bones minimum, B972 clocks out, and the now-famous Neptune 
encounter — not to mention Voyager's Grand Tour — becomes a piece of 
exploration history. 

Back on Earth, the Voyager scientists — data in hand — can continue the 
lengthy but enjoyable task of deciphering the secrets waiting to be found, and 
the solar system will seem evermore a bit smaller. 

TIME: 0 Days, 025:00 
1989/08/25 425 GMT (SCT) 


44546 KM 


Any sufficiently advanced technology is indistinguishable 
from magic . 

Arthur C. Clarke 


You decided long ago that you wanted to go on a Grand Tour of 
exploration, and your destinations were clearly defined. Your ship is 
equipped with the latest in scientific instruments, onboard computers, and 
communications gear. The main problem is that your speed leaving Earth 
is not fast enough (considering the Sun’s gravity) to carry you much beyond 
J upiter — the first of your four destinations. To accomplish the trip, you must 
find a way to increase your speed relative to the Sun. 

A nice fusion drive would do the trick — or maybe a matter/anti-matter 
engine — but these new technologies just aren’t around yet. Fortunately, by 
selecting the proper flight path by each of your destinations, you will be able 
to “steal” some precious speed, fly on to the next more remote destination, 
steal some more speed, and complete your Grand Tour. Knowledge has 
saved the day, and your clever scheme will be called “gravity assist.” 

A Change in Attitude 

The techniques used in the design of planetary missions really did not 
change all that much from the 1920s to about 1960. In the 1920s, Walter 
Hohmann discovered the lowest energy (least departure speed) path be- 
tween any two planets. As shown in Figure 7-1, that path is an ellipse that 
is tangent to the orbits of both the departure planet and the destination 

Planetary mission design primarily consisted of determining the launch 
times for Hohmann transfer ellipses from Earth to the various planets. With 
the rockets that existed by the 1950s, it was thought that it would be a very 
long time before people could send spacecraft beyond the planet Jupiter. The 
energies required for even the “minimum energy” Hohmann ellipses to the 
outer planets were far in excess of what chemical rockets could deliver at 
that time. 

Further complicating matters were the long travel times the Hohmann 
ellipses required. For example, an Earth-to-Pluto Hohmann ellipse re- 
quired a 40- to 50-year one-way travel time. An Earth-to-Neptune Hohmann 
ellipse required a 30-year travel time. It seemed as though not many planets 
would be visited in our lifetimes. 



Figure 7-1. A Hohmann transfer ellipse , tangent to the orbits of both 
the planet one is leaving and the planet one is going to, requires the 
least departure energy or speed. 

In the summer of 1961, a 25-year-old graduate student in mathemat- 
ics, hired as a summer employee at JPL, created a revolution in planetary 
mission design. Michael A. Minovitch showed how to gain extra speed by 
properly selecting the path from planet to planet. 

Minovitch wondered if the gravity field of a planet could be used to 
provide thrust to a spacecraft. Many others before him had thought about 
the effect of planetary gravity fields on passing bodies. But, by 1960, most 
planetary mission designers considered the gravity field of a target planet to 
be somewhat of a nuisance, something to be cancelled out, usually by 
onboard rocket thrust. 

TIME: 0 Days. 0:30:00 
1989/08/25 4:30 GMT (SCT) 

Minovitch was the first to show how to design a 
trajectory to a target planet in such a way that a 
gravity assist could be obtained from that planet to go 
on to another planet. Such a boost could be obtained 
from the second planet to go on to a third planet, etc. 
The only energy required would be the launch from 
Earth to the first planet. All subsequent planets were 


“free.” As an added bonus, due to the gains in speed, the one-way trip times 
to each of the planets beyond the first were significantly reduced. 

By 1962, Minovitch had realized that using the gravity field of Jupiter 
was the key to outer planet exploration. Jupiter is the largest planet and, as 
such, possesses the strongest gravity field. Jupiter could be used to quickly 
slingshot spacecraft to Saturn, Uranus, Neptune, and Pluto, making such 
missions possible for the first time. That same summer, Minovitch realized 
that launch opportunities to the outer planets, via Jupiter, were possible 
from 1962 to 1966 and then recommenced in 1976 until at least 1980. He 
graphically illustrated the trajectory of an Earth-Jupiter-Saturn-Neptune 
Grand Tour, using a 1976 launch. 

In 1 964, Maxwell Hunter publicized Minovitch’s gravity-assist concept 
in an outer planets mission design paper. The next year, Gary Flandro (then 
at JPL, presently founder and president of Wasatch Research, Inc.) designed 
a set of Grand Tour trajectories using the gravity-assist concept, including 
an example of an Earth- Jupiter-Saturn-Uranus-Neptune mission. He pointed 
out that these planets align themselves for this mission only once every ap- 
proximately 176 years. The next set of Earth-launch opportunities would 
occur in 1 976, 1 977, and 1978. This provided the impetus for what ultimately 
became the Voyager Project, including Voyager 2’s Grand Tour of the outer 

Real Applications 

The first application of the gravity-assist concept for planetary explo- 
ration occurred in Mariner 10’s Venus/Mercury mission. The Mariner 10 
spacecraft was launched from Earth in 1973 and travelled directly to Venus 
via a Hohmann transfer ellipse, using the gravity-assist technique at Venus 
in February 1974 to get a boost on to Mercury. At Mercury in March/April 
1974, Mariner 10 received a second gravity assist, which allowed the 
spacecraft to encounter Mercury a second time, in September 1974. A third 
gravity assist was performed at the second Mercury encounter to enable a 
third and final Mercury encounter in March 1975. 

The second application of the gravity-assist concept occurred as a part 
of the Pioneer 11 mission. This spacecraft was originally intended to 
encounter only Jupiter (in 1974), as a precursor to the Voyager-1 and -2 
encounters. However, the opportunity existed to execute a gravity assist at 
Jupiter to go on to Saturn, and Pioneer 11 was able to take advantage of this 
opportunity. Pioneer's gravity-assisted turn was almost 180 degrees, caus- 
ing the spacecraft to travel all the way back across the inner solar system to 
pass closely by Saturn five years later, in 1979. 


Meanwhile, at JPL from 1974 to 1976, Paul Penzo, Audrey 
Sergeyevsky, Joseph Beerer, and Charles Kohlhase evaluated the merits of 
over ten thousand different Voyager trajectories. The objective of the study 
was to maximize the total amount of knowledge that could be gathered from 
the Jovian and Saturnian systems. Of primary interest were Jupiter’s moon 
Io and Saturn’s moon Titan. Each pair of Voyager 1 and 2 trajectories had 
to have at least one close approach to each of these two moons. Additionally, 
the best trajectories had the largest number of close flybys of the remaining 
Jovian and Saturnian satellites. The final trajectories flown are shown in 
Figure 1-4, and include two gravity swingbys at Jupiter, two at Saturn, one 
at Uranus, and one at Neptune. 

Gaining Speed Along the Way 

Gravity assist is created by causing a spacecraft to pass by a planet in 
a carefully controlled manner, as shown in Figure 7-2. A spacecraft may pass 
by the trailing (or leading) hemisphere of a planet. The close passage causes 
two things to occur. First, the spacecraft’s path is bent. Second, the 
spacecraft either gains or loses energy (speed), as described below. 

The bending occurs regardless of whether the spacecraft passes by the 
leading or the trailing he misphere. The direction of the bending is selected 
by picking the proper hemisphere. The amount of bending is controlled by 
picking the closest approach distance to the planet. The bending in the flight 
path occurs both with respect to the planet and with respect to the Sun. 

There is no net change in speed, however, with respect to the planet. The 
spacecraft is in continual free-fall with respect to the planet. Its final speed 
(far after approach) is exactly the same as its initial speed (far before 
approach) with respect to the planet. 

With respect to the Sun, the story is quite different. First note that the 
spacecraft's velocity relative to the Sun is always equal to the spacecraft's 
velocity relative to the assisting planet plus (vector addition) that planet’s 
velocity relative to the Sun. From the point of view of the Sun , when 
comparing the pre- and post-swingby spacecraft velocities, Figure 7-2 shows 
that this results in a net increase in the speed of an outbound (i.e., going 
away from the Sun) spacecraft (and, not shown in the 
figure, in a net slowing down of the planet). Energy 
has been transferred from the planet to the space- 
craft. On the other hand, if an outbound spacecraft 
passes by the leading edge of the planet, from the 
point of view of the Sun, the roles are reversed: the 
spacecraft slows down and the planet speeds up. In 

TIME: 0 Days. 0:35:00 
1989/08/25 4:35 GMT (SCT) 



54921 KM 

P = Probe 

J = Jupiter V P/S nnt 

o __ o out 

With Gravity Assist (5 kg) 

= 16 km/sec 









Jupiter Swingby 

Without Gravity Assist 

Figure 7-2. Passing close by a massive body causes a spacecraft's path to be bent , 
and energy to be exchanged between the spacecraft and body. In the Voyager-1 
Jupiter swingby shown , there is no net speed gain relative to Jupiter ; however, 
Voyager 1 gained 16 km/sec (35.700 mph) relative to the Sun, and Jupiter lost 1 foot 
per trillion years relative to the Sun, causing its orbital period to shrink by nearly 
one nanosecond. 

the case of Voyager 2, this may be seen in Figure 11-6, which dramatically 
shows the behavior of the craft's Sun-relative speed as it swings past each of 
the Jovian giants enroute to escaping from the solar system. These 
principles also apply to gravity-assist applications using the large satellites 
of a planetary system. 

Voyager 1 at Jupiter and Voyager 2 at Jupiter, Saturn, and Uranus 
passed by the trailing hemisphere of the respective planet, gaining speed at 
the expense of each planet. However, Voyager 1 passed (slightly) the leading 
hemisphere of Saturn, and Voyager 2 will pass (slightly) the leading 
hemisphere of Neptune. In these two cases, the spacecraft slowed down and 
the planets speeded up. 

Diving for Triton 

Neptune is Voyager 2’s last planet. There being no next planet to seek 
(Pluto is not reachable; refer to Figure 6-2), Voyager 2 is not limited to 
passing Neptune through any particular gravity-assist corridor, and can 
instead concentrate on Neptune’s large moon, Triton. Triton is as interest- 
ing to many planetary scientists as Neptune is. Triton is large enough to 
have an atmosphere. Its surface temperature and pressure are close to the 


triple point of nitrogen, raising the possibility of nitrogen clouds, frozen 
nitrogen pools , and snow/ice on the surface. 

In 1980, Andrey Sergeyevsky discovered that there was indeed a way 
to pass closely by both Neptune and Triton, thereby maximizing the scien- 
tific return from each. The means was a final application of the gravity- 
deflection concept. The spacecraft would pass very close to Neptune (within 
4850 kilometers of the cloud tops) in order to bend its path by about 45 
degrees to pass close by Triton 5.2 hours later (see Figure 6-1.) The close 
passage of Neptune occurs near its North Pole, and is just barely on the 
leading hemisphere. Voyager 2 will slow down slightly (and Neptune will 
speed up even more slightly) as a result of this final gravity assist. 

The Solar System is Ours 

Before Minovitch applied his gravity- assist design concept, planetary 
spacecraft were limited to visiting Mercury, Venus, Mars, and Jupiter. 
Using gravity assist, missions to all the planets are possible. Spacecraft have 
travelled directly to Venus, Mars, and Jupiter. Mercury, Saturn, Uranus, 
and (as of the summer of 1 989) Neptune have been visited via gravity assists. 
A mission to Pluto, using a Jovian gravity assist, will undoubtedly occur 

The next planned applications of the gravity-assist technique involve 
the use of planetary moons to provide the assist to planetary orbiters. 
Planetary systems that have large moons can be toured by using the gravity 
of the large moon(s) to deflect the spacecraft’s orbit each time around. 

The Galileo orbiter of Jupiter will perform ten gravity assists at Io, 
Europa, Ganymede, and Callisto, creating ten very close encounters of the 
latter three moons, and an additional three relatively close encounters. 
Galileo is due to launch in the fall of 1989, and will perform gravity assists 
at Venus (in 1990), and at Earth (1990 and 1992), before arriving at Jupiter 
in 1995. 

On the drawing boards is a Saturn orbiter gravity-assisted touring 
mission. Forty gravity assists at Saturn’s large moon Titan are planned for 
the Cassini spacecraft, leading to four very close passages and twenty-six 

TIME: 0 Days. 0:40:00 
1989/08/25 4:40 GMT (SCT) 

relatively close passages of other Saturnian moons. 
Cassini is due to launch in 1996, and will perform 
gravity assists at the Earth (in 1998), and at Jupiter 
(in 2000), before arriving at Saturn in late 2002. 

For more information on Galileo and Cassini, 
see Chapter 16. 



60371 KM 

If there are two or more ways to do something ; and one of 
those ways can result in catastrophe , someone will do it. 

Captain Murphy to Major Stapp, 1949 1 


In Chapter 3, you were introduced to the variety of efforts conducted 
here on Earth that are essential to the success of a mission as complex as this 
one, far away at Neptune. And in Chapter 4, you learned about the 
remarkable robot called Voyager 2 that was designed to carry it out, in 
conjunction with the facilities of Earth Base. 

People created, designed, and built all of this technology, and it is 
people who plan, execute, and monitor the mission. Perhaps this is why 
things don’t always proceed as expected. There are goblins lurking every- 
where — on Earth, on Voyager 2, and even at Neptune — and they are 
constantly scheming to trip us up, to interrupt the wealth of data streaming 
back from our remote, electromechanical emissary. 

In this chapter, we will examine what is called contingency planning. 
A contingency is an event or situation that, if it occurs, can cause a reduction 
in the quantity and/or quality of returned Voyager data. Since the funda- 
mental purpose of Voyager 2 and all that supports it is to return these data 
to Earth, then the purpose of contingency planning is to preclude goblins in 
the first place, and to preserve the data return when some goblins do sneak 
through our defenses. 

None of the contingencies we get concerned about has a particularly 
high chance of happening; most are estimated to be less likely than one 
chance in twenty on an annual basis. Nevertheless, a chance is a chance, and 
it is prudent to take some actions in many cases. 

In spite of everyone’s contingency planning efforts, unexpected goblins 
can strike suddenly. Things simply go wrong and we are faced with a 
potential loss of valuable data. Once the Voyager system is attacked, a 
diligent effort ensues to determine the goblin’s precise hiding place and to 
arrest and survey the inflicted damage. A thorough study of the precise 
cause of the problem is then conducted, leading to a recommended solution 
(or “fix”) for that problem. If possible, methods and procedures are devised 
which essentially inoculate the Voyager system against future reappear- 
ances of the goblin that precipitated the problem. 

‘This is the original wording for Murphy's Law. See the end of this chapter for the story . . . 




Figure 8-1. Goblins , like the spacecraft-munching Great Galactic Ghoul (once 
sketched in fun , when early spacecraft seemed to experience problems when they 
reached certain distances from the Sun), are lurking everywhere. With well-laid 
contingency plans, however, we hope to thwart their evilintentions. (Artist: G.W. 

Let’s review some Voyager history first, then look at these goblins more 
closely, and see what is done to outwit them. 

Past Skirmishes: The Aches and Pains of Voyager 2 

The Voyager contingency planning effort started well before launch, 
during the mission and spacecraft design phase, and continues to this day. 
Staying one step ahead of the goblins is a vigil that can never cease until each 
Voyager meets its ultimate demise. 

To no one’s surprise, goblins have been encountered all during this 
project (starting as early as the launch-through-Earth-departure flight 
sequence), as they are during all complex projects. Some have been stopped 
in their tracks by the various contingency planning 
provisions, but many have sneaked by. Nevertheless, 
it is a tribute to the keepers of Voyager 1 and Voyager 
2 that both spacecraft are operating well after nearly 
12 years in space. In fact, in many ways, both are 
operating with more capability than they had at 
launch, as the next chapter will show. 

TIME: 0 Days. 0:45:00 
1989/08/25 4:45 GMT (SCT) 



Past skirmishes with the goblins have inflicted their wounds on both 
spacecraft; fortunately (since it is going to Neptune), Voyager 2 is probably 
the healthier of the two, considering that both of its FDS memories are still 
usable. Table 8-1 summarizes the lasting aches and pains that it has 
suffered along the way on its Grand Tour of the outer solar system, and 
includes the “treatment” prescribed and administered to circumvent the 
injury. In spite of the failed units, degraded components, and sporadic 
anomalies, we are coping with the problems. Most importantly, all ten of 
Voyager 2’s science instruments are functional, as is the radio equipment. 
Thus, barring a severely damaging onslaught by the goblins, we are confi- 
dent that investigations in all eleven science experiment categories will be 
successfully carried out at Neptune. 

Table8-1. Voyager 2 has felt its share of aches and pains over the years since launch, 
but Earth's doctors (the Flight Team) have taken admirable care of their distant 
patient . 

Voyager 2 Health and Status Actions Taken by Project/Comments 

• Overall condition 

• No serious problems 

• All science instruments 

• Except for consumables, spacecraft is operating 
with more capability than at launch 

• Expecting investigations in all science 
experiment categories at Neptune 

• Failed components 

• Receiver 1 

• Receiver 2 signal lock circuit 

• Using special "best-lock frequency" tests and 
procedures. Carefully managing Voyager power 
and thermal states; "backup mission loads" 
stored on board to provide science return 
should Receiver 2 fail 

• Degraded components 

• One memory word lost in FDS A; 
256-word block lost in FDS B 

• Azimuth actuator seized at 
Saturn; okay since 

• Some PPS filter and analyzer 
wheel selections lost 

• Decrease in narrow-angle camera 
vidicon cathode emission 

• Weakening IRIS interferometer 
and neon cathode emission 

• PWS and LECP sensitivity 

• Spurious resets in PRA electronics 

• No longer using these memory locations 

• High-rate slewing banned; other slewing 
limited; using special actuator health tests; 
on-the-shelf R951 CCS load design 

• No longer using these selections 

• Imposed constraints on total on time, on-off 
cycles, and diagnostic data readout 

• Imposed special thermal conditioning constraints 

• Implemented special sequencing and 
and procedural fixes 

• Special autonomous reset sensing/correction 
routine active on board 


The Goblins and Their Mischief 

Goblins generally come in three varieties. There are those in the 
“human element” category, spawned by our innate tendency to err. Whether 
they are called “operator error,” “cockpit error,” or “indeterminate cause,” 
they all can be traced to the same source: we humans simply make mistakes 
once in a while. Another type is the more common “glitch”, when seemingly 
perfect hardware or software suddenly hiccups and goes into an unantici- 
pated operating mode, or even worse, stops working altogether. Sometimes 
the glitch is more gradual and drawn out, in which case it may be called a 
“graceful degradation.” A third species of goblin is “acts of nature” — those 
natural environmental effects and hazards that are commonly associated 
with space travel: radiation, temperature extremes, dust and debris, 

physical uncertainty, and the like. Natural factors on Earth such as stormy 
weather and earthquakes influence the contingency plan as well. 

The goblins tend to concentrate their mischief in three areas: the 
Computer Command Subsystem (CCS) sequence development process, the 
telecommunications link between Earth and V oy ager , and on V oy ager itself. 

As you found in Chapter 3, the CCS load generation process is lengthy, 
requiring the concerted efforts of dozens of people using sophisticated 
computer programs and simulators to take the explicit sequence requests 
and convert them into a complex string of Is and Os that will tell Voyager 2 
to do exactly what we want it to do, and nothing else. Such a system is open 
ground for the goblins, prone to human error. A tightly woven network of 
sequencing rules, reviews, checks and balances, and constraint checking is 
required throughout the sequence development process to ensure that a 
near-perfect sequence is generated. This activity — which could be described 
as building software that must work the first time — makes extensive use of 
people, computers, and seemingly endless coordination meetings. 

Voyager 2 is so far away from us now that managing telecommunica- 
tions — commanding the spacecraft and receiving and routing its teleme- 
try — is an imposing and challenging task. The DSN antennas must be 
precisely pointed, as must the spacecraft. All data rates and modes must be 
matched. State-of-the-art DSN receiving gear must be maintained. Mar- 
gins must be added into the performance envelopes to 

TIME: 0 Days, 0:50:00 _ .. _ _ . . . ,, , , 

1989/08/25 4:50 gmt (sct) account for possible degradations in signal level due 

to pointing offsets and weather. Timing is often 
critical. Again, this is open territory for goblins of 
many types. 

And, in spite of its stupendous string of suc- 
cesses to date, Voyager 2 is still a vulnerable piece of 


71523 KM 


technology, susceptible to the hazards of space and, perhaps more impor- 
tantly, aging. Some resources on board are consumable, such as the 
hydrazine fuel for the thrusters. Other components most certainly have 
limited operating lifetimes, such as the radio transmitter tubes, or subas- 
semblies with moving parts such as the digital tape recorder and scan 
platform actuators. Electronic devices and switches, though generally quite 
reliable, can fail permanently — and quite unexpectedly. These spacecraft- 
borne problems can often take a considerable effort to understand, since all 
we have to look at is the overall spacecraft performance and limited 
telemetry data. A repair visit to Voyager 2 is simply beyond our reach. 

Anomalies more often than not arise from changing the normal way of 
doing things. As you will find in the next chapter, new challenges at Neptune 
have demanded some changes in the way we operate Voyager 2 and its 
support network. These changes, in turn, are a potentially rich source of 
goblins, and have required a fresh look at the contingency plan for the 
mission. Much of this plan is based on contingency preparations completed 
prior to the 1986 Uranus encounter, adjusted somewhat for Neptune to 
reflect the upgrades in telecommunications capability, enhancements to 
spacecraft performance, different encounter characteristics, and the ramifi- 
cations of being 50 percent farther from the Sun. 

Before we see how the contingency plan addresses all of this, let’s first 
look closely at perhaps the most significant driver of this plan: environ- 
mental hazards in the Neptune system. 

Taking the Plunge 

Voyager 2’s north polar trajectory places the spacecraft closer to 
Neptune than any other outer planet encounter to date, and thus potentially 
closer to various environmental influences there (see Figure 8-2). Assess- 
ment of the risks posed by this near encounter with N eptune required models 
of its ring system, magnetosphere, atmosphere, and obscuration periods — 
times when the spacecraft cannot precisely sense the Sun or stellar (star) 
reference because either Neptune, its ring-arc system, or Triton is in the 
way. As Voyager 2 plunges through the Neptunian system, slam-dunking its 
way toward Triton, all four environmental goblins have the potential of cata- 
strophically disrupting the encounter, so something must be done to protect 
our fragile craft. 

As you learned in Chapter 2, the ring system at Neptune is apparently 
quite different than those observed at other gas giants, mainly because of the 
evidence for patchy ring arcs rather than continuous rings. In addition to the 
ring-arc region, past experience at other outer planets and current theory 



41.000 km 
56,500 km 

67.000 km 

0 25 50 

Thousands of km 

Figure 8-2. We are looking perpendicular to Voyager-2’s path through the 
environmental hazard zones . Table 8-2 summarizes the periods during which we 
think the goblins have the greatest chance to do their mischief. 

suggest that a diffuse, thin disk of ring material is likely inside, and possibly 
outside, the inferred ring-arc system. Such diffuse rings were observed by 
Voyager in and around the main rings at Jupiter, Saturn, and Uranus, and 
were penetrated (out beyond the main rings) without damage to the space- 
craft at the latter two planets. 

The Project’s overriding concern about rings focuses on the possibility 
that Voyager 2 might impact ring particles and suffer a significant degrada- 
tion of a subsystem or instrument capability, or worse yet, a catastrophic 
failure. This concern is heightened by the fact that Voyager 2 first crosses 
the ring plane before most of the high-value encounter science is collected. 

Extensive analyses of the ring issue have led to a consensus on one 
important point: the probability of actually hitting a narrow ring arc with 
Voyager 2 — even if we wanted to — is very low (less 
than 1 percent). The concern really is, then, the 
unseen diffuse disk of material, especially during the 
inbound ring-plane crossing, which is closer to the 
planet. Unfortunately, no direct observations of this 
purported sheet will be obtainable using ground- 
based telescopes, and the chance of getting a defini- 

TIME: 0 Days, 0:55:00 
1989/08/25 4:55 GMT (SCT) 



77167 KM 

Table 8-2. Periods of Greatest Vulnerability 

• Diffuse Disk of Ring Particles 
Inbound Ring-plane Crossing 
Outbound Ring-plane Crossing 

-lh 8m to -44m 
+lhllm to +lh 49m 

• Radiation 

Peak Radiation Inbound 
Peak Radiation Outbound 
25%, 50%, 75%, 99% Dosage 

-50h, -30m, +40m, +60m 

-60m to -20m 

+40m to +50m 

• Atmosphere 

Drag, Heating and Arcing 

-5m to+5m 

• Obscuration Zones 



-lh to +2h 
+5h 20m to +6h 10m 

tive observation with Voyager-2 pictures is considered extremely low until 
after Neptune closest approach, when the sheet might be visible in forward- 
scattered sunlight, much like when dust or dew is visible on your car 
windshield when driving into the Sun. 

By choosing a trajectory just outside where we think the ring-arc region 
ends (assuming equatorial rings, as shown in Figure 2-9), we can avoid the 
most likely locale for concentrations of this diffuse material. Such a path 
thus directs the threat of goblins to the possible region of diffuse material 
outside the ring-arc region — a threat found to be idle at Saturn and Uranus 
because the dust particles in this region were so small and dispersed. If 
Neptune has polar rings, however, as a few scientists have suggested, 
Voyager 2 could cross an inner diffuse sheet — a threat of some concern. 

Another Neptune system environmental hazard is radiation. Severe 
radiation can damage Voyager's science instruments and subsystem hard- 
ware, degrade performance, create calibration difficulties, cause onboard 
timing errors, cause frequency shifts, and inject unwanted noise into various 
data paths. The Flight Data Subsystem (FDS) is considered vulnerable to 
this radiation as well. Predicting the magnitude of these effects is an 
imprecise science even when the radiation environment is known exactly, 
much less when next to nothing is known about it, as is the case for Neptune. 
The planetary magnetospheres that trap this radiation are so large that 
these effects are generally independent of small trajectory changes, so at 
Neptune, this is one goblin we must live with. 

To evaluate this hazard to Voyager 2, a model of energetic electrons was 
required. Voyager 1 experienced several temporary and permanent hard- 


ware degradations and failures when subjected to radiation as it passed near 
Jupiter, and this radiation was subsequently correlated with such high- 
energy electrons. Voyager 2 is passing even closer (relatively) to Neptune, 
so a potentially greater radiation hazard would be likely if Neptune’s 
magnetosphere matched Jupiter’s. However, according to present under- 
standing, Neptune’s magnetosphere is more likely to resemble that of 
Uranus in size and intensity. On the path Voyager 2 will take, the present 
Neptune radiation model predicts a peak radiation “flux” level 57 percent as 
strong as the Voyager-1 Jupiter level, and a total radiation dosage of only 3 
percent of the Voyager-1 Jupiter level; both levels are considered safe. 

The next environmental goblin — the atmosphere — can produce effects 
that have barely been considered during past Voyager encounters. Voyager 
2 is aimed for a region on the fringe of Neptune’s atmosphere, where effects 
such as loss of attitude control (due to drag), heating, and corona discharge 
between high-voltage components are possible, centered around Neptune 
closest approach. With the proper aim and some clever operational tricks, 
however, Voyager 2 will pass beyond the grasp of this goblin, as we shall soon 

Calculation of obscuration periods is primarily a geometric problem 
once the desired trajectory is selected and the various positional relation- 
ships of the Neptunian system are generally understood. The uncertain 
nature of the postulated ring-arc system adds a special twist to these 
calculations, since the individual arcs may partially block the Sun or stars 
and confuse Voyager’s sensors. 

Outwitting the Goblins 

With so much operational experience behind us (including an ample set 
of anomalies), it is not terribly difficult to anticipate most plausible anoma- 
lies that might occur, and even some that aren’t so plausible. The challenge 
facing Voyager personnel, therefore, is not in generating a long list of 
potential goblins, but in formulating an effective defense against them 
without overextending available resources. There are lots of goblins, but we 
simply do not have unlimited supplies of people, time, computer resources, 
and money with which to mount our defense, so these 
resources must be balanced between the efforts re- 
quired to develop the nominal mission plan and those 
required for the contingency plan. The end result is 
that only some of the possible contingency provisions 
get implemented, and calculated yet conservative 
risks are taken. 

TIME: ODays. 1:00:00 
1989/08/25 4:60 GMT (SCT) 



As mentioned before, the contingency planning efforts for the Neptune 
encounter started first with an update of the plan devised for Uranus. 
Goblins that could lead to a science loss were then postulated and ranked to 
determine contingency work priorities. The ranking exercise considered 
criteria such as probability of contingency occurrence, the abruptness of the 
occurrence, the data loss potential, the recovery time required if unprepared 
for the anomaly, and the effectiveness of the proposed contingency protec- 
tion. This entire effort took most of a year to complete (mid-1987 through 

The primary goal of the contingency protection is to ensure an adequate 
defense against goblins in the critical near encounter CCS load, B951 . This 
two-day load includes closest approaches of Neptune and Triton and associ- 
ated Sun and Earth occupations, most of the top-priority science observa- 
tions for the entire four-month encounter, and all of the environmental 
hazard periods. Once this protection is provided, remaining project re- 
sources are allocated to define other protective measures for events with 
unique timing or placement requirements, such as maneuvers and geome- 
try-unique science observations. Generic protection techniques applicable 
to all CCS loads (including cruise loads) have also been devised. 

So, keeping in mind the nominal sequence of events outlined in Chapter 
6, here are some of the more interesting examples of contingency provisions 
that have been implemented (in addition to those in Table 8-1) to keep the 
goblins subdued and in check. 

LOSS OF ONE CCS. All CCS loads can be modified in about three 
days to make them execute out of only one CCS. The penalty is that a 
moderate to significant amount of science must be excised from the original 
load to allow it to fit in the limited memory of one CCS unit. 

LOSS OF ONE FDS. A very clever set of FDS programming features 
and CCS load design rules allows each CCS load to execute virtually without 
modification if an entire FDS is lost. Earth-based operators would be busy 
for a few days loading in the Neptune Single Processor Program (NSPP) and 
getting everything set up for this mode of operation, but most of the expected 
science and engineering data would still be returned. This capability is new 
for Neptune. 

tioned above that a path has been selected for the Neptune encounter that 
avoids most of the ring hazard by sending the inbound spacecraft through, 
at most, a diffuse disk of ring material — small dust particles, really. In spite 
of this conservative targeting strategy, the sensitive optical surfaces on 
Voyager’s scan platform instruments (cameras, IRIS, PPS, and UVS) could 


still be abraded by these particles as they slam into Voyager at 25 km per 
second. The contingency measure taken to counteract this possibility is to 
point the instruments away from the incoming ring dust while Voyager 
passes through the closer, first ring-plane crossing. 

To address the radiation hazard, special precautions are being taken. 
The spacecraft actually has two internal clocks — one in the FDS and one in 
the CCS. Each controls different types of sequence activities, but only the 
FDS can “fall behind” when given a dose of radiation. However, as some CCS 
events occur relative to FDS clock signals, it was decided to adopt two 
precautions. Key CCS events in CCS load B951 have been moved several 
seconds past the FDS “frame-start” signal, and a special onboard software 
routine has also been designed to “re-synchronize” the two clocks every now 
and then during the critical B951 near-encounter sequence. 

For the atmospheric hazard, a conservative targeting strategy has been 
chosen as well: the planned path is expected to be at about 4400 km above 
the “detectable” atmosphere — albeit only 600 km beyond the reach of the 
nearest goblins able to cause some spacecraft problems even in the extremely 
rarefied outer fringes of Neptune's upper atmosphere. Nevertheless, for 
added protection near closest approach, Voyager’s thrusters will be config- 
ured to push a bit harder each time they fire to enhance the craft’s stability 
if it should be subjected to a small amount of atmospheric drag. 

Even in the extreme case in which the atmosphere is found to extend 
farther out from Neptune than even our worst projections, something can be 
done to salvage the B951 science. If the larger atmosphere is detected early 
enough during approach, Voyager’s path could actually be moved out from 
the preplanned trajectory with Trajectory Change Maneuver (TCM) B19 or 
TCM B20, and the B951 load could be updated to accommodate the small 
changes in instrument pointing and event timing induced by this deviation 
from the nominal plan. This clever contingency feature (which could also be 
used for ring- arc avoidance) is appropriately called the “mini-bailout” option. 

The final environmental goblin, obscuration, is defeated very easily: 
instead of using the Sun or a star as its reference during these periods, 
Voyager 2 is configured to rely on its onboard gyroscopes and thrusters for 
attitude reference and control, eliminating the need 
altogether for tracking the Sun and stars. 

PLATFORM ACTUATOR. The actuator that con- 
trols scan platform azimuth motion seized unexpect- 
edly on Voyager 2 during the Saturn encounter. 
Though the performance for both actuators has been 

TIME: ODays, 1:05:00 
1989/08/25 5:05 GMT (SCT) 


88513 KM 


flawless since (following some diagnostic tests and implementation of special 
operating procedures), the project still considers this a plausible goblin, 
more so for the azimuth actuator than for elevation. Now the scan platform 
is treated with kid gloves, and all slewing activity is carefully planned and 
monitored. Slewing at the highest rate — which was the rate being used 
extensively before the seizure at Saturn — is now forbidden. 

Periodic tests of actuator health, called Torque Margin Tests (TMTs), 
are performed to see if a goblin is lurking around the corner. The TMTs 
employ very short pulses of electrical current to nudge the actuator along 
while spacecraft specialists on Earth look for signs of excess friction or 
erratic motion. A period of science observation time in CCS load B921 has 
been earmarked for removal if analysts find evidence of actuator degrada- 
tion and the project subsequently decides to perform a contingency TMT 
followed by healing exercises on the balky actuator. 

For the azimuth actuator, an option exists to disable its slewing should 
it continue to perform poorly after the healing exercises, and then conduct 
the critical B951 load with elevation slewing only. For such a scenario, a 
special version of B951 that is compatible with elevation-only slewing has 
been created and will sit “on the shelf,” ready to be used if this goblin strikes. 
This special version of B951 (referred to as R951 because the spacecraft must 
roll to compensate for the lack of azimuth motion) is the only special version 
of B951 for the Neptune encounter; at Uranus, three such loads were 
developed for the comparable high-value CCS load. 

A MISSED MANEUVER. For a variety of reasons, it is possible 
(though very unlikely) to miss the preplanned execution of one of Voyager 2’s 
critical approach trajectory correction maneuvers. In the case of TCM B18, 
a backup opportunity has been planned about one week after the nominal 
time. In fact, this period is essentially the same period of time mentioned 
above for the scan platform contingency window in B921 . Either goblin is 
unlikely, and both appearing at the same time is highly unlikely. Thus, we 
can use the same window of time as a dual-purpose contingency slot without 
taking on unnecessary risk. 

LOSS OF DIGITAL TAPE RECORDER. This piece of equipment 
has no backup on the Voyager spacecraft, so if it fails, all data recording 
capability would be lost. To insulate ourselves from the effects of such a 
goblin, a few things can be done. First, an appropriate mix of recorded and 
real-time (i.e., non-recorded, “live”) data is planned, whenever possible. 
(This is not possible during the occupation periods, since direct communica- 
tion with Earth is precluded when Voyager is behind Neptune or Triton.) 
Next, for many observations, more than the minimum desired amount of 




Figure 8-3. We don't expect a disaster (during the encounter period) like the San 
Fernando earthquake of 1971, but f with odds of 2 percent per year for a major quake 
in southern California , some contingency planning is wise. (Courtesy of Caltech 
Earthquake Engineering Library.) 

data is planned into the sequence, so that if some are lost — for whatever 
reason — the scientists will still be happy. And, to accommodate a situation 
where bad weather on Earth engulfs the receiving DSN site, playback of the 
recorded data is backed up by a redundant playback, just in case the first is 
missed or degraded. 

plans for this goblin! The estimated probability of it poking its annoying nose 
in our business is 2 percent a year. If a major earthquake (or any other 
natural disaster, for that matter) should strike southern California, commu- 
nications links between JPL and the remote DSN sites would most likely be 
disrupted for at least several hours to possibly many days. And if the 
encounter computer loads are not uplinked to Voy- 
ager in time, we’re in big trouble. To ensure that this 
happens as scheduled for the most critical CCS load, 
a copy of the uplinkable series of 1 s and Os for this load 
will be stored on a special computer disk at each 
applicable DSN site, ready to be used for the uplink 
process if JPL is knocked out. The version of B951 

TIME: 0 Days, 1:10:00 
1989/08/25 5:10 GMT (SCT) 



94197 KM 

stored on the disk might be slightly out of date, but sending that to the 
spacecraft is far better than having nothing at all up there. 

* * * * 

Well, there you have it. We all hope — and yes, expect — that the 
encounter will proceed smoothly and, if history is any indicator, it will. 
Nevertheless, having a solid contingency plan in place still helps everyone 
think a little clearer and sleep a bit better. 


In 1949, U.S. Air Force Captain Murphy (who keeps his first name 
secret to maintain some privacy) was in charge of a group seeking ways to 
increase the probability of a pilot surviving an aircraft crash through better 
seat design. Understanding how much of a deceleration the human body 
could withstand was one of the first tasks for the team. At the site of what 
is now Edwards Air Force Base in California, an officer-physician named 
Major John Stapp was acting as his own guinea pig by riding on a rocket sled 
at great speeds, followed by extremely fast stops. Captain Murphy supplied 
the critical deceleration sensors. 

After some lower-speed preliminary trials, Major Stapp decided to take 
a risk and push the limits: he rode the sled up to 600 miles per hour, then 
came to a dead stop in less than two seconds, subjecting himself to a 
deceleration of over 40 times the normal force of gravity. He was left in pretty 
bad shape. What's worse, Murphy’s sensors were installed in the only 
orientation of several that would lead to useless data, so Stapp had to do it 

After Murphy realized what had happened, he looked at the Major and 
said, “If there are two or more ways to do something, and one of those ways 
can result in catastrophe, someone will do it.” Stapp looked at Murphy and 
uttered, “That’s Murphy’s Law.” 

The next day at a press conference, Major Stapp altered Murphy’s 
wording significantly, resulting in the more common version of Murphy’s 
Law: If anything can go wrong, it will. 

Murphy still likes his original version better ... 

From an interview with Murphy in 
The Beach Reporter , Manhattan 
Beach, California, 1983 July 28, 
by Tom Adams. 


TIME: ODays, 1:1 5:00 
1989/08/25 5:15 GMT (SCT) 


99881 KM 


Fortune sides with him who dares . 



Imagine yourself at an international speedway watching a conference 
among designer, mechanic, and driver on how to win a long distance 
endurance race with a twelve-year-old racing car. Typically, the driver and 
other members of the team will brainstorm together in a cycle of design, test, 
and simulation to guarantee any projected new performance of their race car. 

To have any hope of success against stiff odds, the team will try to 
invent new ways to squeeze extra performance out of their aging machine by 
the use of special engine tune-ups, new driving techniques, and by making 
special efforts to conserve fuel, tires, and other consumables to avoid 
frequent change-outs at the speedway pit stop. 

The analogy of winning an international competition using an old 
racing car illustrates how the Voyager Flight Team has prepared for another 
race — to Neptune and beyond. The benefit of upgrading an aging (but well- 
designed) one-ton robot is to get a first-class look at the outermost giant 
planet of the solar system. Furthermore, this will be achieved at a modest 
additional cost beyond that spent for the primary Jupiter/Saturn and 
Uranus missions. When compared to the billion or so dollars that a newly 
designed outer planets mission would cost, not to mention the necessity of 
waiting well into the next century for results, the Voyager-2 mission to 
Neptune is a bargain! 

While use of the international speedway’s repair facilities allows our 
race-car team to update its old machine to compete in the race on Earth, the 
Voyager Flight Team cannot call the Voyager spacecraft back to Earth from 
its distant location. However, the Flight Team can reprogram Voyager’s 
onboard computers to effect new strategies to win the race to Neptune and 
beyond. Reconfiguring the spacecraft’s computer memories is somewhat 
analogous to choosing a new racing driver with more experience (i.e., with a 
better-performing brain and superior motor skills). 

Maintaining a Strong Signal 

With Neptune at nearly six times the Earth-to- Jupiter distance of 779 
million km (483 million mi), the maximum data rate that can be received at 
Earth (and still just “hear” the signal above the noise level) would fall by a 



factor of nearly thirty-six (due to the square-of-distance penalty 1 ) unless the 
Voyager Flight Team could pull some rabbits out of a hat. 

At Jupiter’s distance of five astronomical units (AU) the maximum data 
rate was 115,200 bits per second, at Saturn (10 AU) the maximum data rate 
was 44,800 bits per second, while at Uranus (19 AU) it was 29,900 bits per 
second. Plans for Neptune (30 AU) call for a maximum data rate of 21,600 
bits per second. A major upgrade of the DSN's large antennas and the 
arraying of tracking antennas, as described below, allow us to arrest the 
natural fall of the signal strength to only half (rather than 1/9) that at Saturn 
when Voyager 2 reaches Neptune. 

To meet the needs of Voyager at Neptune, as well as to enhance other 
future missions, the DSN undertook an ambitious program to convert its 
three large 64-m antennas to 70-m dishes. This was accomplished by tearing 
down and discarding all of the old metallic surface plates and structural 
outrigger beams, and then installing a totally new outer support structure 
along with precision surface plates that, once in place, could be adjusted to 
submillimeter accuracy. Holographic alignment techniques were intro- 
duced that permitted sharp focusing of the short wavelength (3-cm) radio 
signals. Together, the larger surface area and alignment and calibration 
techniques have yielded an improvement in signal strength of 55 percent for 
each 70-m antenna. 

The Voyager Proj ect has called upon additional resources beyond the 
NASA/JPL-operated DSN for data acquisition at the Neptune encounter. As 
was done for the Uranus encounter, the DSN is again teaming up with the 
Australian government’s Parkes radio astronomy 64-m antenna operated by 
the Commonwealth Scientific and Industrial Research Organization (CSIRO). 
Three antennas (one 70-m and one 34-m) of the DSN facility in Canberra will 
combine signals (array) with the Parkes antenna via a 320-km (200-mi) 
microwave link. By simultaneously tracking Voyager from up to three 
antennas during the Neptune encounter period, the DSN and Parkes radio 
observatories will achieve a significant increase in the combined signal 
strength, which is roughly proportional to the combined surface areas of all 
arrayed antennas, to help defeat the square-of-distance penalty. 

TIME: 0 Days. 1 :20:00 

1989/08/25 5:20 GMT (SCT) l A formula known as the inverse-square law is used to estimate 

the decrease in signal strength: radio signals weaken accordingto 
the ratio of the squares of the distances from the transmitter to the 
receiver. At the time of Jupiter flyby in 1979, for example, Jupiter 
was at about 5 AU, while Neptune will be at 30 AU. Therefore, 
5 2 /30 2 = (1/6) 2 = 1/36. In other words, the signals received from 
Voyager 2 at Neptune will be about 36 times weaker than those 
that were received from the Voyagers at Jupiter. 


105561 KM 




However, by far the greatest signal strength improvement for Neptune 
will result from arraying the National Radio Astronomy Observatory’s 
(NRAO) Very Large Array (VLA) antennas (twenty-seven 25-m dishes) near 
Socorro, New Mexico, with the Goldstone, California, DSN complex. The 
received signal power when the VLA is arrayed with a 70-m DSN antenna 
will be nearly three times greater than that received by the 70-m antenna by 
itself. Figure 9-1 shows a portion of the VLA antenna complex that 
heretofore has never been used for receiving telemetry data from an inter- 
planetary spacecraft. Data will be relayed in real-time to the Goldstone site 
via a satellite microwave link between the VLA and the DSN. 

Lastly, an additional enhancement has been made to the N eptune radio 
science experiment through a cooperative venture with the Japanese space 
agency permitting the use of their 64-m Usuda antenna on the day of 
encounter for the non-real-time combining of radio science data. 

To take maximum advantage of these new signal reception capabilities, 
the onboard software in the Flight Data Subsystem (FDS) was completely 

Figure 9-1. One of Voyager's new “hearing aids ” consists of twenty-seven 25-m 
steerable radio antennas that form the Very Large Array located in the solitude of 
a New Mexico desert. These same antennas have been scanning the heavens, 
studying galactic radio sources, also listening intently for faint radio signals in the 
NRAO’s Search for Extra-Terrestrial Intelligence (SETI) Program. 


reprogrammed for more optimum data rates, data formats, and picture- 
editing capabilities. 

Discarding Unnecessary Picture Data 

Neither the arraying of antennas over Australia nor the addition of the 
VLA to Goldstone’s antennas can completely overcome the square-of-dis- 
tance penalty. The Voyager Flight Team has therefore developed a clever 
scheme to pre-process the imaging data to reduce the total number of bits 
required to transmit a TV picture. This scheme was successfully introduced 
at Uranus encounter and will again be used at Neptune. They have used a 
special software routine known as Image Data Compression (IDC) in the 
onboard FDS backup computer, newly reconfigured for this task. JPL’s own 
Robert F. Rice designed and developed the IDC technique. 

Uncompressed Voyager TV images contain 800 lines, 800 dots (pixels) 
per line, and 8 bits per pixel (to express one of 256 gray levels). This means 
that every uncompressed TV image requires over five million bits. However, 
much of the information in a typical television image of a planetary system 
is frequently dark space or low-contrast cloud features. Therefore, by 
counting only the differences between adjacent pixel grey levels rather then 
the full 8-bit values, IDC can reduce by 60% or more the number of bits that 
characterize each image and thus reduce the time needed to transmit a 
complete TV image from Neptune to Earth. 

As a rule, the reconstructed compressed image will be indistinguish- 
able from the uncompressed image, as the IDC scheme loses no information 
for low-contrast scenes. Even for scenes with rapidly changing pixel 
intensities, such as the Saturn ring image shown in Figure 9-2, only minor 
line clipping occurs near the left- and right-hand edges of the frame. 

More Accuracy for Fewer Bits 

Another trick devised to beat the square-of-distance penalty is the use 
of an onboard “experimental” Reed-Solomon (RS) data encoder. For those of 
you who know about secret codes used to hide information in the context of 
spy thrillers, it may be reassuring to learn that there are also codes designed 
to preserve the “truth” of information. Data sent to 
the Earth pass through a plasma that may phase- 
modulate the signals with noise, i.e., turn a “correct” 
0 bit into a “wrong” 1 bit, or vice versa. 

Encoding these data has a price, and that paid 
for the old Golay encoding algorithm (used at Jupiter 
and Saturn) was one code bit overhead for every data 


TIME: ODays, 1:25:00 
1989/08/25 5:25 GMT (SCT) 



Figure 9-2. Each Voyager image at Jupiter and Saturn contained five million binary 
bits of data. To cope with the reduced data rates available from remote Uranus and 
Neptune , onboard data compression “differences” adjacent pixel brightness levels 
to return only two million bits per picture. 

bit (100 percent). The new RS encoding scheme reduces this overhead to 
about 20 percent. In addition, it reduces the number of bit errors from 5 in 
100,000 to only 1 in a million! 

The field of information theory is much too esoteric for this Guide. 
Many have tried to approach C.E. Shannon’s famous information capacity 
limit for a data channel. Figure 9-3 shows a rare gathering of four modern- 
day pioneers whose clever mathematical coding schemes have made impor- 
tant strides in efficient information transmission. 

Taking Good Pictures in Feeble Light Levels 

There is yet another penalty imposed on the Neptune encounter by the 
square-of-distance law. Reflected solar visual radiation from the Neptunian 
system is received by the spacecraft instruments at very reduced light levels 
(some 900 times fainter than at Earth). Thus, longer exposure times are 
required to gather the faint light, but this makes smear (picture-blurring) of 
rapidly moving targets, such as Triton or the Neptune ring-arcs, much more 
of a problem than it was at Jupiter, Saturn, or Uranus. The problem facing 
Voyager engineers is somewhat analogous to a situation confronting a 
photographer in a dimly lit room without a flash. To offset the required long 
exposure times, he must steady the camera on a tripod, use very sensitive 
film, or open the camera aperture. If the subject is moving, the photographer 
must smoothly pan the camera to 'Track” the target, much as a WWII tail 
gunner in a B1 7 had to do during combat missions. 




Figure 9-3. The Uranus encounter in 1 986 brought together four pioneers responsible 
for the NASA-standard Reed-SolomonfViterbi coding system and the Rice data 
compression scheme. From left to right are Robert Rice of JPL f Andrew Viterbi of 
Qualcomm , Gustave Solomon of Hughes Aircraft Company ; and Irving Reed of 
USC. (Photograph: Rick S. Austin.) 

Once some preliminary estimates of required exposure times at Nep- 
tune had been calculated, the Project realized that neither the ground-based 
nor spacecraft-borne software programs were prepared to handle precise 
exposures beyond about 15 seconds. Therefore, a reprogramming and 
spacecraft testing effort was undertaken to provide a continuity of exposure 
times up to approximately one minute, as well as to permit multiples of 48- 
second increments beyond the original 0- to 15-second exposure interval. 

For the Saturn and Uranus encounters, a technique was developed to 
use the spacecraft’s gyroscopes to smoothly execute image motion compen- 
sation (IMC) turns to track selected targets during the near-encounter 
phase. Communications are broken off during IMC, since Voyager’s antenna 
is moved off of Earthline, and all IMC data must be recorded. 

A new capability termed nodding image motion 

TIME: 0 Days, 1:30:00 

1989/08/25 5:30 gmt (sct) compensation (NIMC) has been developed for the 

Neptune encounter. NIMC permits the spacecraft to 
remain nearly Earth-pointed while turning slightly, 
) \ shuttering a frame, and turning back to Earth- point, 

r — ' This means the pictures can be transmitted directly 


116899 KM 



Figure 9-4. A WWII gunner had to slew his guns to track 
an enemy aircraft. In a similar manner , a photographer 
must pan his camera to avoid smeared pictures of moving 
subjects. (Courtesy of the Boeing Company.) 

to the ground in real-time without the need for intermediate storage on the 
digital tape recorder (DTR). The nodding motion of the spacecraft is 
accurately controlled by precisely calibrating the small attitude-control 
thrusters, and then programming the onboard computer to fire the thrusters 
a predetermined number of times as needed to turn the spacecraft at the 
desired IMC rate. This is followed by a reset to the initial position in 
preparation for the next frame. Meanwhile, of course, the camera must be 
re-pointed between frames to account for the ever-changing target direction 
and spacecraft orientation. 

Finally, as shown in Figure 9-5, another new capability planned for 
Neptune is called maneuverless image motion compensation (MIMC), 
whereby the scan platform itself is turned slowly (albeit somewhat jerkily 
because of the actuator design, which uses a stepper motor) relative to the 
spacecraft during an observation. Because of the unsteady turning motion, 
this panning technique has limited utility and is used only for first-order 


correction of rapidly moving targets that would otherwise appear hopelessly 
smeared. It was necessary to test a new scan platform turning rate for the 
sequencing process in order to obtain the proper MIMC rate for the chosen 

As Voyager cruises along in a zero-gravity environment, the start-stop 
motion of its tape recorder can add more jiggle to the spacecraft’s natural 
limit cycle motion. To reduce these types of disturbances, the Voyager Flight 
Team has devised new software that fires the spacecraft thrusters to offset 
the tape recorder speed change whenever the tape recorder starts or stops. 
In addition, the pulse duration of the spacecraft’s tiny attitude-control 

Classical IMC 

Entire spacecraft 
turns to track 
target — breaks 
with Earth 

Nodding (NIMC) 

Spacecraft "nods" to 
track target — stays 
on Earthline — 
cameras repointed 
between images 

Maneuverless (MIMC) 

Move scan platform 
only — elevation only 

TIME: ODays, 1:35:00 
1989/08/25 5:35 GMT (SCT) 

Figure 9-5. Who says you can ’ t teach an old dog new tricks 1 
To reduce picture smear when using long exposure times 
to handle the low light levels at Neptune , three different 
techniques have been developed to “track” the target. 


122556 KM 


thrusters has been reduced to provide a steadier spacecraft and thus 
decrease the number of blurred images. 

Diagnosing the Health of the Actuators 

Voyager 2 has performed remarkably well during the past twelve years, 
with only a few major hardware problems. For example, 102 minutes after 
Voyager 2 flew by Saturn closest approach on August 25, 1981, the scan 
platform stopped during a high-rate azimuth slew to a new target. This was 
believed to be due to a temporary seizure in the gear train of a small actuator 
(electric motor). Since that time, the Voyager Flight Team has devised a 
strategy to conserve platform usage and to operate the small drive actuators 
at lower speeds. 

In addition, a clever method of checking the health of the platform 
actuators was developed to indirectly measure the amount of friction in the 
gear train. This Torque Margin Test (TMT) varies the duration or “width” 
of the electrical pulses to the stepper motor that drives the actuator gear 
train (see Figure 4-5). Even a healthy actuator would fail to move the 
platform if the stepper motor pulse width were reduced so low that it could 
not generate a minimum torque to overcome the normal frictional losses in 
the system. 

The TMT strategy is to use these reduced pulse widths and note 
whether the platform slews at reduced or intermittent rates. The health of 
an actuator is gauged by the minimum pulse width required to slew the 
platform at the normally expected rate. An increase in this minimum pulse 
width may indicate degradation due to increased frictional losses from an 
unhealthy actuator. The TMT will be used during the Neptune encounter 
period to monitor the actuator’s performance (see Chapters 6 and 8). 

Faster Response From the “Old” Robot 

Since Saturn, the Voyager engineers have also added a new capability 
to perform higher-rate spacecraft turns about the roll axis in time-critical 
periods of the encounter. This capability to roll at 0.3 deg/sec may be used to 
conserve the worrisome actuator gears by selectively substituting spacecraft 
roll maneuvers for scan platform azimuth movement, and to prepare for a 
contingency backup in the remote possibility of another scan platform 

The Voyager engineers, knowing that the previous gyro-controlled IMC 
maximum rates were too slow to track the moon Miranda during the mad 
dash past Uranus, reprogrammed the onboard computers to overcome this 
hurdle. Instead of the former capability (at Saturn) of 70 deg/h (the sum of 


all three axis rates), Voyager will be able to perform gyro-drift turns as fast 
as 120 deg/h for each axis, simultaneously. Because of this, we are looking 
for some nice, sharp images of Triton. 

Big Changes in the Deep Space Network 

Waiting to capture the Voyager data from Neptune will be the multi- 
million-dollar Mark IVA configuration of the Deep Space Network, as devel- 
oped for the Uranus encounter. Nine DSN antennas, located in three Deep 
Space Communications Complexes (DSCCs) around the world, are sched- 
uled to be controlled and operated according to an advanced concept of 
command, communications, and control (C 3 ) that uses new microprocessors 
and software distributed over a Local Area Network (LAN). 

Each individual antenna and its co-located electronics will operate 
unattended except for maintenance. A Complex Monitor Control (CMC) 
operator configures electronic resources, located in the new Signal Process- 
ing Center (SPC) at each complex, for each antenna at the complex. No 
longer will each antenna individually require a complete set of dedicated 
electronics to fulfill downlink telemetry, command, ranging, or long-baseline 
navigation functions. 

Another recent capability is that faulty equipment should be quickly 
replaced by the CMC operator if required, thus preserving vital science data 
during critical moments of the encounter. Like any new large and complex 
system implementation, the Mark IVA has had its teething problems, but 
they were resolved by the start of the Uranus encounter. 

Each flight project such as Voyager will be assigned to one or more Link 
Monitor Control (LMC) console operators. While the CMC operator config- 
ures the electronics for each antenna to support a particular schedule, the 
LMC operator (see Figure 9-6) assures the required data processing support 
for individual spacecraft, and controls antenna performance via the LAN. 
Typically, one LMC operator is available for each antenna at a complex. 

The “arraying” of antennas is a scheme whereby all antennas receive 
the Voyager signals, and the separate subcarrier signals are combined to 
achieve a greater signal-to-noise ratio (SNR). This Mark IVA capability to 
array antennas for additional SNR is, of course, vital 
1 989/08^25 5:40 gmt (sci) to the success of the Voyager encounter at Neptune. 


128202 KM 


Figure 9-6. Not to be forgotten are the dedicated efforts of DSN personnel at remote 
locations. The Link Monitor Control operators , shown above at the Madrid DSCC, 
are vital in capturing , saving , and relaying scientific data back to JPL. 

The Bottom Line 

This overview provides some idea of why the Voyager Flight Team 
hopes to win the race to Neptune and beyond: because it, JPL, and NASA are 
determined to provide, at minimum cost, a manyfold increase in the science 
knowledge of this giant planet by continually upgrading the capabilities of 
the aging Voyager spacecraft and the supporting facilities at Earth Base. 




TIME: ODays, 1:45:00 
1989/08/25 5:45 GMT (SCT) 



133839 KM 

The disposition of a fly's wings or of the feelers of a snail is 
sufficient to confound you . 



The Voyager mission was officially approved in May 1972, has received 
the dedicated efforts of many skilled personnel for nearly two decades, and 
has returned more new knowledge about the outer planets than had existed 
in all of the preceding history of astronomy and planetary science. And the 
two Voyager machines are still performing like champs. 

It must come as no surprise that there are many remarkable, "gee- 
whiz” facts associated with the various aspects of the Voyager mission. 
These tidbits have been summarized in this chapter in appropriate catego- 
ries. Several may seem difficult to believe, but they are all true and accurate. 

Overall Mission 

1. The total cost of the Voyager mission from May 1972 through the 
Neptune encounter (including launch vehicles, nuclear-power-source 
RTGs, and DSN tracking support) is 865 million dollars. At first, this 
may sound very expensive, but the fantastic returns are a bargain 
when we place the costs in the proper perspective. It is important to 
realize that: 

(a) on a per-capita basis, this is only 20 cents per U.S. resident per 
year, or roughly half the cost of one candy bar each year since 
project inception. 

(b) the daily interest on the U.S. national debt is a major fraction of 
the entire cost of Voyager. 

2. A total of 11,000 workyears will have been devoted to the Voyager 
project through the Neptune encounter. This is equivalent to one- 
third the amount of effort estimated to complete the great pyramid at 
Giza to King Cheops. 

3. A total of five trillion bits of scientific data will have been returned to 
Earth by both Voyager spacecraft at the completion of the Neptune 
encounter. This represents enough bits to encode over 6000 complete 
sets of the Encyclopedia Brittanica, and is equivalent to about 1000 
bits of information provided to each person on Earth. 



Figure 10-1. On a U.S. resident per-capita basis, Voyager is a remarkable bargain 

at 20 cents per year. 

4. The sensitivity of our deep-space tracking antennas located around 
the world is truly amazing. The antennas must capture Voyager 
information from a signal so weak that the power striking the antenna 
is only 10 16 watts (1 part in 10 quadrillion). A modern-day electronic 
digital watch operates at a power level 20 billion times greater than 
this feeble level. 

Voyager Spacecraft 

1. Each Voyager spacecraft comprises 65,000 individual parts. Many of 
these parts have a large number of “equivalent” smaller parts such as 
transistors. One computer memory alone contains over one million 
equivalent electronic parts, with each spacecraft containing some five 
million equivalent parts. Since a color TV set contains about 2500 
equivalent parts, each Voyager has the equivalent electronic circuit 
complexity of some 2000 color TV sets. 

Like the HAL computer aboard the ship Discov- 
ery from the famous science fiction story 2001: 
A Space Odyssey , each Voyager is equipped 
with computer programming for autonomous 
fault protection. The Voyager system is one of 
the most sophisticated ever designed for a deep- 


TIME: ODays. 1:50:00 
1989/08/25 5:50 GMT (SCT) 




Figure 10-2 . Both Voyagers have returned five trillion bits of science data since 
launch , equivalent in information bits to that needed to encode 6000 sets of the 
Encyclopedia Brittanica. 

space probe. There are seven top-level fault protection routines, each 
capable of covering a multitude of possible failures. The spacecraft can 
place itself in a safe state in a matter of only seconds or minutes, an 
ability that is critical for its survival when round-trip communication 
times from Earth stretch to several hours as the spacecraft journeys to 
the remote outer solar system. 

3. Both Voyagers were specifically designed and protected to withstand 
the large radiation dosage during the Jupiter swing-by. This was 
accomplished by selecting radiation-hardened parts and by shielding 
very sensitive parts. An unprotected human passenger riding aboard 
Voyager 1 during its Jupiter encounter would have received a radia- 
tion dose equal to one thousand times the lethal level. 

4. The Voyager spacecraft can point its scientific instruments on the scan 
platform to an accuracy of better than one-tenth of a degree. This is 
comparable to bowling strike-after-strike ad infinitum , assuming that 
you must hit within one inch of the strike pocket every time. Such 
precision is necessary to properly center the narrow-angle picture 


whose square field-of-view would be equivalent to the width of a 
bowling pin. 

5. To avoid smearing in Voyager's television pictures, spacecraft angular 

rates must be extremely small to hold the cameras as steady as 
possible during the exposure time. Each spacecraft is so steady that 
angular rates are typically 1 5 times slower than the motion of a clock's 
hour hand. But even this will not be quite steady enough at Neptune, 
where light levels are 900 times fainter than those on Earth. Space- 
craft engineers have already devised ways to make Voyager 30 times 
steadier than the hour hand on a clock. 

6. The electronics and heaters aboard each nearly one-ton Voyager 
spacecraft can operate on only 400 watts of power, or roughly one- 
fourth that used by an average residential home in the western United 

7. A set of small thrusters provides Voyager with the capability for 
attitude control and trajectory correction. Each of these tiny assem- 
blies has a thrust of only three ounces. In the absence of friction, on 
a level road, it would take nearly six hours to accelerate a large car up 
to a speed of 48 km/h (30 mph) using one of these thrusters. 

8. The Voyager scan platform can be moved about two axes of rotation. 
A thumb-sized motor in the gear train drive assembly (which turns 
9000 revolutions for each single revolution of the scan platform) will 
have rotated five million revolutions from launch through the Neptune 
encounter. This is equivalent to the number of automobile crankshaft 
revolutions during a trip of 2725 km (1700 mi). 

9. The Voyager gyroscopes can detect spacecraft angular motion as little 
as one ten-thousandth of a degree. The Sun’s apparent motion in our 
sky moves over 40 times that amount in just one second. 

TIME: ODays, 1:55:00 


The tape recorder aboard each Voyager has 
been designed to record and playback a great 
deal of scientific data. The tape head should not 
begin to wear out until the tape has been moved 
back and forth through a distance comparable to 
that across the United States. Imagine playing 


145080 KM 




Figure 10-3. Each Voyager spacecraft consists of about 5,000,000 equivalent 
electronic parts — comparable to some 2000 color TV sets. 

a two-hour video cassette on your home VCR once a day for the next 22 
years, without a failure. 

11. The Voyager magnetometers are mounted on a frail, spindly, fiber- 
glass boom that was unfurled from a two-foot-long can shortly after the 
spacecraft left Earth. After the boom telescoped and rotated out of the 
can to an extension of nearly 1 3 meters (43 feet), the orientations of the 
magnetometer sensors were controlled to an accuracy better than two 


1 . Each Voyager used the enormous gravity field of Jupiter to be hurled 

on to Saturn, experiencing a Sun-relative speed increase of roughly 
35,700 mph. As total energy within the solar system must be con- 
served, Jupiter was initially slowed in its solar orbit — but by only one 
foot per trillion years. Additional gravity-assist swing-bys of Saturn 
and Uranus were necessary for Voyager 2 to complete its Grand Tour 
flight to Neptune, reducing the trip time by nearly twenty years when 
compared to the unassisted Earth-to-Neptune route. 


2. The Voyager delivery accuracy at Neptune of 100 km (62 miles), 
divided by the trip distance or arc length traveled of 7,128,603,456 km 
(4,429,508,700 mi), is equivalent to the feat of sinking a 3630-km 
(2260-mi) golf putt, assuming that the golfer can make a few illegal fine 
adjustments while the ball is rolling across this incredibly long green. 

3. Voyager’s fuel efficiency (in terms of mpg) is quite impressive. Even 
though most of the launch vehicle’s 700-ton weight is due to rocket fuel, 
Voyager 2’s great travel distance of 7.1 billion km (4.4 billion mi) from 
launch to Neptune results in a fuel economy of about 13,000 km per 
liter (30,000 mi per gallon). As Voyager 2 streaks by Neptune and 
coasts out of the solar system, this economy will get better and better! 


1. The resolution of the Voyager narrow-angle television cameras is 
sharp enough to read a newspaper headline at a distance of 1 km (0.62 

2. Pele, the largest of the volcanos seen on Jupiter’s moon lo, is throwing 
sulfur and sulfur-dioxide products to heights 30 times that of Mount 
Everest, and the fallout zone covers an area the size of France. The 
eruption of Mount St. Helens was but a tiny hiccup in comparison (ad- 
mittedly, Io’s surface-level gravity is some six times weaker than that 
of Earth). 

3. The smooth water-ice surface of Jupiter’s moon Europa may hide an 
ocean beneath, but some scientists believe any past oceans have 
turned to slush or ice. In 2010 : Odyssey Two , Arthur C. Clarke wraps 
his story around the possibility of life developing within the oceans of 

TIME: 0 Days. 2:00:00 
1989/08/25 6:00 GMT (SCT) 

The rings of Saturn appeared to the Voyagers as a dazzling necklace 
of 10,000 strands. Trillions of ice particles and car-sized bergs race 
along each of the million-kilometer-long tracks, with 
the traffic flow orchestrated by the combined gravita- 
tional tugs of Saturn, a retinue of moons and moon- 
lets, and even nearby ring particles. The rings of 
Saturn are so thin in proportion to their 171,000-km 
(106,000-mi) width that, if a full-scale model were to 
be built with the thickness of a phonograph record, 


the model would have to measure four miles from its inner edge to its 
outer rim. An intricate tapestry of ring-particle patterns is created by 
many complex dynamic interactions that have spawned new theories 
of wave and particle motion. 

5. Saturn’s largest moon Titan was seen as a strange world with its dense 
atmosphere and variety of hydrocarbons that slowly fall upon seas of 
ethane and methane. To some scientists, Titan, with its principally 
nitrogen atmosphere, seemed like a small Earth whose evolution had 
long ago been halted by the arrival of its ice age, perhaps deep-freezing 
a few organic relics beneath its present surface. 

6. The rings of Uranus are so dark that Voyager’s challenge of taking 
their picture was comparable to the task of photographing a pile of 
charcoal briquettes at the foot of a Christmas tree, illuminated only by 
a 1-watt bulb at the top of the tree, using ASA-64 film. And Neptune 
light levels will be less than half those at Uranus. 

The Future 

1. The solar system does not end at the orbit of Pluto, the ninth planet. 
Nor does it end at the heliopause boundary, where the solar wind can 
no longer continue to expand outward against the interstellar wind. It 
extends over a thousand times farther out where a swarm of small 
cometary nuclei, termed Oort’s Cloud, is barely held in orbit by the 
Sun’s gravity, feeble at such a great distance. Voyager 1 passed above 
the orbit of Pluto in May 1 988, and Voyager 2 will pass beneath Pluto's 
orbit in August 1990. But even at speeds of over 35,000 mph, it will 
take nearly 20,000 years for the Voyagers to reach the middle of the 
comet swarm, and possibly twice this long for them to pass the outer 
boundaries of cometary space. By this time, they will have traveled a 
distance of two light-years, equivalent to half of the distance to 
Proxima Centauri, the nearest star. 

2. Barring any serious spacecraft subsystem failures, the Voyagers may 
survive until the early twenty-first century, when diminishing power 
and hydrazine levels will prevent further operation. Were it not for 
these dwindling consumables and the possibility of losing lock on the 
faint Sun, our tracking antennas could continue to “talk” with the 
Voyagers for another century or two! See Table 12-1 for a listing of 
lifetime limiting factors. 


Don’t forget to see Chapter 12 for more amazing facts about the flights 
of the two Voyager spacecraft as they silently coast towards other stars 
several millennia into the future. 


156279 KM 

Are thy wings plumed indeed for such far flights 1 

Walt Whitman 


Prepare yourself to enter into a new realm of distance, speed, and time 
as measured by the journeys of the Voyager spacecraft. Here you do not think 
in terms of the units used for an automobile trip, say from Los Angeles to New 
York, a distance of 2800 miles with a driving time of 56 hours at an average 
speed of 50 mph. But rather we will use metric units. Thus, the same road 
trip becomes 4500 kilometers at a speed of 80 km/h. More importantly 
though, you must vastly expand the distance dimensions used to measure 
the flights of spacecraft that have trip durations of many years. 

Because of the large distances between planetary bodies, it is usually 
convenient to express these distances in terms of Astronomical Units (AU), 
where 1 AU is defined as the mean distance of the Earth from the Sun and 
equals 149,600,000 kilometers (or about 93 million miles). To put this 
distance into proper perspective, consider the fact that it would take you 212 
years of nonstop driving at 80 km/h (50 mph) to travel just 1 AU. Even a 
supersonic transport traveling at Mach 2.5 (1900 mph) would take almost 6 
years nonstop. 

The Grand Tour 

Figure 11-1 shows the paths followed by the two Voyager spacecraft, as 
well as by Pioneers 10 and 11, as they spiral outward from their Earth- 
launch points. Both Voyagers have had close encounters with Jupiter and 
Saturn, but only Voyager 2 continued on to Uranus and Neptune. This 
occurred for two reasons. First, the unique “Grand Tour” alignment of 
Earth, Jupiter, Saturn, Uranus, and Neptune occurs for only three consecu- 
tive launch years out of every 176 years, and 1977 was one of those golden 
planetary moments. And second, Voyager 1 arrived at Saturn first and 
successfully scanned the top-priority moon Titan, freeing the later-arriving 
Voyager 2 from the Titan obligation, thereby allowing it to be targeted on to 
Uranus and Neptune. 

The Voyager spacecraft were both launched in 1 977, and the Pioneer 1 0 
and 11 spacecraft were launched in 1972 and 1973, respectively. These four 
spacecraft are the very first that will escape the gravity of our solar system 
as they continue their unending journeys into the Milky Way galaxy at 
speeds of “only” a few AU per year. For Voyager 1, the 3.5 AU/yr departure 


Figure 11-1. This is an ecliptic plane projection of the Voyager and Pioneer flight 
paths. All will escape from the solar system , but the faster-moving Voyagers will 
win the race. Planet and spacecraft positions are shown in 2000 A.D. 

speed is nearly 60,000 km/h (37,200 mph), while for Voyager 2 the departure 
speed is just overt 53,400 km/h (33,100 mph). Although this may seem fast 
by terrestrial standards, you’ll soon realize that it is excruciatingly slow by 
interstellar standards. If you want a little rule for estimating how far the 
Voyagers will be from our Sun in some future year, use 76.34 + 3.50 (Year - 
2000) for Voyager 1 and 59.75 + 3.13 (Year - 2000) for Voyager 2. . . andyou’ll 
have the approximate distance in AU. 

Figure 11-2 presents a three-dimensional view of the Voyager space- 
craft trajectories. As a consequence of encountering Titan prior to Saturn 
closest approach, Voyager 1 passed somewhat “beneath” Saturn, being 
deflected upwards, north of the ecliptic plane at an angle of about 35 degrees. 

After the Neptune encounter, Voyager 2 will depart south of the ecliptic 
plane at an angle of approximately 48 degrees. This departure condition 
results from Project plans to obtain a 40,000 km (25,000 mi) flyby of the 
satellite Triton after skimming over the northern polar regions of Neptune 
(being deflected downward) at an altitude of just 4850 km (3010 mi) above 
the cloud tops (100 millibars). 

TIME: 0 Days, 2:10:00 

The Great Escape 

From Figure 11-1 it can be seen that Pioneer 10 
has a head start on the other spacecraft, having 
passed the orbit of Neptune in 1985 at a solar radius 


161863 KM 


Figure 1 1-2. Gravity assist has deflected the two Voyagers out of the ecliptic plane. 
Flying “under” Saturn, Voyager 1 was lofted above the ecliptic at a 35° angle. 

of 30 AU. However, Voyager 1 was the first to cross over Pluto’s eccentric 
inclined orbit in 1988, at a distance of about 29 AU, when Pluto’s orbit was 
inside that of Neptune’s. Pioneer 11 crossed over the Uranian orbit slightly 
before Voyager 2’s 1986 encounter, but Voyager 2 will be the first to reach 
(and encounter) Neptune. Because of the significant speed advantage of the 
two Voyager spacecraft, they will gradually out-distance the Pioneers in the 
twenty-first century. 

A very challenging future goal is for the Voyager Project to reach the 
heliopause boundary (see Figure 12-1) with spacecraft that are operational 
at a distance of 50 to 150 AU. 

However, the spacecraft won’t have enough power to operate much 
beyond the year 2017, when Voyagers 1 and 2 will be at distances from the 
Sun of 138 AU and 113 AU, respectively. Far beyond the heliopause, at the 
very edge of our solar system, the Voyagers will pass through Oort’s Cloud 
of cometary nuclei. However, at the cloud’s great distance of at least 63,000 
AU (about 1 light-year), the Voyagers will not arrive for another 20,000 


Voyager 2 at Neptune 

Relative to an observer on Neptune, Voyager 2 is approaching from 
generally the Sun’s direction at a speed of about 60,000 km/h (37,500 mph). 
Meanwhile, Neptune is orbiting the Sun at a mean radius of 30 AU with a 
speed in excess of 19,500 km/h (12,100 mph), but the giant planet still takes 
165 years to complete just one orbit. (Neptune has not completed one orbit 
since its discovery 143 years ago!) By the Neptune closest approach time of 
04:00 GMT on August 25, 1989, the spacecraft will have travelled a distance 
of more than 47 AU (4,359,300,000 mi) along its heliocentric path since 
leaving Earth 12 years earlier. 

Neptune is truly a giant planet, with a diameter of 49,600 km (30,800 
mi), compared to Earth at 12,800 km (7,900 mi). Neptune is nearly four times 
the diameter of Earth and 57 times larger in volume than Earth. It is 
significant to note that the path of Voyager 2 must pass within 4850 km of 
the cloud tops of Neptune in order to provide the proper gravitational 
deflection to pass close to Triton. 

Another interesting view of the Neptune encounter is shown in Figure 
6-1, showing north polar passage of Neptune by the spacecraft, followed by 
both Earth and solar occupations by Neptune and any existing rings. At the 
time of closest approach, a radio signal being sent from the spacecraft to 
Earth will take 4 hours and 6 minutes to reach Earth. This means the data 
transmitted at the time of closest approach will be seen back on Earth at 
08:06 GMT or 1 :06 AM PDT, Friday, August 25, 1 989. The most intense time 
for media coverage will run from about 12 hours before to 50 hours after 
Neptune closest approach (allowing for recorder playbacks). 

There are just two known satellites of Neptune. Triton, the larger, has 
an estimated diameter of 3000 km (1860 miles), nearly 500 km smaller than 
Earth’s Moon. Triton’s exact size is very uncertain, however, and could be 
as small as 2200 km (1360 mi), or as large as 5000 km (3100 mi). Triton 
revolves in a retrograde direction around Neptune once every 5.9 days, in an 
orbit inclined about 20° to Neptune’s equatorial plane. Triton travels at a 
speed of about 1 5,800 km/h (9800 mph), relative to N eptune, in its orbit about 
the gas giant. 

The smaller satellite, Nereid, has an estimated 
diameter of 800 km (500 mi). Nereid takes 360 days 
(about one year) to revolve around Neptune, in an 
orbit inclined 30° to Neptune’s equator, at a speed of 
about 4000 km/h (2500 mph) relative to Neptune. 
Nereid’s orbit is highly eccentric, so its distance from 
Neptune varies greatly. Its semi-major axis is 


TIME: 0 Days. 2:15:00 
1989/08/25 6:15 GMT (SCT) 

Figure 11-3. Voyager 2 comes no closer than about 4.6 million km (2.9 million mi) 
to Nereid. 

5,510,000 km, or 14 times the distance from the Earth to the Moon (see 
Figure 11-3). 

Key Events , Distances , and Speeds 

Table 11-1 provides a list of trajectory events during the Voyager-2 
approach and encounter with Neptune, including a few key science links (see 
also the end of Chapter 2, portions of Chapter 6, and the acronym short 
descriptions in Chapter 19) as well. The times shown for the latter refer to 
the link start times. 


Table 11-1. Neptune Encounter Trajectory Events 


Event Time 

Time from Neptune 
Closest Approach 

Distance (km) 
(from center 
of event body) 

Start Observatory Phase 

6/05/89 06:42 



8/01/89 19:56 


Start Far-Encounter Phase 

8/06/89 08:42 



8/15/89 09:42 





Start Near-Encounter Phase 


Nereid best imaging 


Triton longitudinal imaging 


20 Neptune radii* 

8/24/89 20:37 



Search for ring arcs 


Uplink Late Stored Update 


Ring occultation of Sigma-Sagittarii 


Nereid closest approach 

8/25/89 00:23 



10 Neptune radii 




Triton longitudinal imaging 


Ascending node 




Roll spacecraft to +61 deg. 


Neptune closest approach** 




Enter Neptune umbra (Sun occultation) 04:06 



Enter Neptune Earth occultation 




Start limbtrack maneuver 


Exit Neptune Earth occultation 




Exit Neptune umbra (Sun occultation) 04:56 



Descending node 




End limbtrack maneuver 


Roll spacecraft to Alkaid 


Triton imaging (VTCOLOR) 


Triton mapping (VTMAP) 


10 Neptune radii 




Triton highest resolution (VTERM) 


Triton occults B Canis Majoris 




Triton closest approach 




Enter Triton Earth occultation** 




Enter Triton Sun occultation 




Exit Triton Earth occultation 




Exit Triton Sun occultation 




Roll spacecraft to Canopus 


Triton crescent imaging 


20 Neptune radii 




Start Post-Encounter Phase 


End Post-Encounter Phase 


* Times given are the nominal start times and may vary by a few 
minutes due to ongoing updates. 

**One-way light time is 246 minutes at Neptune closest approach. 
+ Assumes a 100 mbar equatorial radius value of 24,781 km. 

++ Assumes Triton radius of 1500 km for Sun and Earth occulta- 





173001 KM 

Finally, many news people like to know spacecraft distance (how far) 
and speed (how fast) at almost any time the urge strikes them to want these 
facts. For “8-place precision,” the Voyager Navigation Team must be 
consulted and, of course, the requester must specify the “relative-to” body for 
distance or speed. 

However, for those who would be content with approximate values, 
they may find Figures 11-4 and 11-5 quite useful. After all, in the time it 
takes to say “Voyager 2 is four billion, four hundred and twenty-four million, 
nine hundred and forty-four thousand, six hundred and eighty kilometers 
from Earth,” the spacecraft has moved 1 60 km (100 mi) and Earth has moved 
300 km (185 mi)! And, if the listener wants to repeat back the number for 
verification, well . . . you get the idea. 

Finally, Figure 11-6 illustrates the speed of Voyager 2 (relative to the 
Sun) as it leaves the already fast-moving Earth, picks up gravity-assist (see 
Chapter 7) speed gains from Jupiter, Saturn, and Uranus, loses a little speed 
by passing Neptune slightly on its leading hemisphere, and heads on out of 
the solar system on its escape trajectory. The massive Sun, though pulling 
back on the spacecraft, cannot slow Voyager's speed enough to prevent the 
craft's escape. 

Figure 11-4. These ranges should be accurate enough for most information 
purposes. In fact , in the time it takes to quote an "< eight-place " figure , all of the 
bodies have moved a few hundred kilometers. 





8 46 

1~ 44 

"“<3 42 



<N -43 oq 

^ cd OO 

II 36 






6/4/89 11 18 25 7/2 9 16 23 30 8/6 13 20 27 9/3 10 17 24 10/3/89 

Calendar Date 

Figure 11-5 . Truly , everything is relative. When you ask how fast Voyager 2 is 
moving , you must specify " relative to what.” Remember that 1 km/sec is roughly 
2237 mph! 

Radial Distance from Sun (AU) 

TIME: 0 Days, 225:00 

Figure 11-6. Following gravity-assist speed gains from 
Jupiter, Saturn, and Uranus, the Sun's massive gravity 
cannot halt Voyager 2 's escape from the solar system. The 
tiny craft speeds up, then slows down, as it swings by each 
gas giant, but (as discussed in Chapter 7) only experiences 
net speed changes relative to the Sun. 



178556 KM 

All I ask is a tall ship and a star to steer her by. 

John Masefield 


Following the Neptune encounter, the Voyager spacecraft will continue 
to travel outward from the Sun. As the influence of the Sun’s magnetic field 
and solar wind grow weaker, both craft will eventually pass out of the 
heliosphere and into interstellar space. 

The Voyager Interstellar Mission 

The two Voyager spacecraft will continue to be tracked on this outward 
journey, just as the Pioneer spacecraft are. This extended mission is called 
the Voyager Interstellar Mission (VIM), and officially begins on January 1, 
1990. The VIM will extend the NASA Planetary Program’s exploration of the 
solar system far beyond the neighborhood of the outer planets. The 
spacecraft escape trajectories are shown in Figures 11-1 and 11-2. The 
mission objectives of the VIM are: 

(a) To investigate the interplanetary and interstellar media, and to 
characterize the interaction between the two, and 

(b) To continue the successful Voyager program of ultraviolet astronomy. 

Fields and Particles Investigations During the VIM 

During the VIM, the Voyager spacecraft will explore the nature of the 
fields and particles environment of the solar system and will search for the 
heliopause. Figure 12-1 gives our present view of what this environment 
looks like. It shows the heliosphere, the inner heliosphere shock front, and 
the heliopause, as well as the trajectories of the Voyager and Pioneer 
spacecraft, all relative to the direction of the interstellar wind. 

The solar wind is made up of particles streaming outward from the Sun. 
These are mainly protons, electrons, and alpha particles (i.e., helium nuclei). 
The solar wind particles are typically boiled off from the solar corona. They 
become less dense with increasing distance from the Sun, ultimately becom- 
ing a low-density wind of outward-moving particles. The density in the wind 
as it reaches the Earth varies markedly with time, sometimes being as low 
as one atom per cubic centimeter (cc), and sometimes as high as 1000 atoms 
per cc. The high-density situations are associated with the occurrence of 
flares and other disturbances in the solar atmosphere. 





Figure 12-1 . The heliopause is at the outermost extent of the solar wind . Beyond 
the heliopause lies the interstellar wind. 

At the inner heliospheric shock front there is a sharp drop in the 
velocity of the solar wind, which goes from being supersonic to being 
subsonic. Outside the shock front, it is more of a solar breeze, and can be 
deflected away from the heliopause. 

The heliopause is at the outermost extent of the solar wind, where the 
interstellar medium restricts the outward flow of the solar wind and confines 
it within a magnetic bubble called the heliosphere. 

Beyond the heliopause lies the interstellar wind. Hopefully the space- 
craft will still be functioning when they penetrate the heliopause to sample 
the interstellar medium, allowing measurements to be made of interstellar 
fields and particles unaffected by the solar plasma. 

Unfortunately, it is not known just where the heliopause is. It is 
believed to be located between 50 and 150 AU from 

TIME: 0 Days, 230:00 . , 

1989/08/25 6:30 gmt (sen the Sun in the direction that the Voyagers are travel- 

v . ing. It is also not known just how abruptly the density 

of material changes upon crossing the heliopause. 

Locating the heliopause is a major objective of 
the VIM. Finding out the nature of the interstellar 
( TjN medium beyond the heliopause is another major ob- 


184101 KM 


jective. And, of course, while the Voyagers are traveling outward to the 
heliopause, they will be mapping the solar wind. 

Typical scientific objectives to be addressed during the VIM (by the 
indicated investigations) are to: 

(a) Understand how the solar wind changes with distance from the Sun 

(b) Observe the variation in the distant interplanetary medium during 
the 11 -year solar cycle (MAG, PLS, LECP, CRS, PWS); 

(c) Study latitudinal variations in the interplanetary medium (MAG, 

(d) Search for low-energy cosmic rays (LECP, CRS); 

(e) Observe and describe the Sun's magnetic field reversal (MAG, PLS, 

(f) Understand how particles from the solar wind are accelerated and/or 
brought into thermodynamic equilibrium (MAG, PLS, LECP, PWS, 

(g) Search for evidence of interstellar hydrogen and helium from the 
interstellar wind (UVS, PLS, LECP, CRS); 

(h) Observe and describe the inner heliospheric shock front and the 
heliopause (MAG, PLS, LECP, CRS, PWS); 

(i) Characterize the subsonic solar wind beyond the inner heliospheric 
shock front (MAG, PLS, LECP, CRS, PWS); 

(j) Explore the local interstellar medium beyond the heliopause (MAG, 

(k) Search for radio emissions from the Sun and solar wind (PRA, PWS); 

(l) Search for radio emissions generated at the interface between the 
solar wind and the interstellar wind (PRA, PWS); and 

(m) Search for low-frequency radio emissions emanating from outside the 
heliosphere (PRA, PWS). 

Ultraviolet Astronomy During the VIM 

The Voyager UVS instruments are unique among present spaceborne 
ultraviolet spectroscopic observatories in that they can observe a region of 
the extreme ultraviolet spectrum (shortward of 1200 angstroms) not pres- 
ently covered by any other spacecraft. 

The only other NASA spacecraft capable of observing in this spectral 
region was Copernicus (OAO-3), which is now defunct. This region of the UV 
spectrum, from 5 00 to 1200 angstroms, is a relatively unexplored region, 
where we are able to observe some of the more energetic phenomena in the 
Universe. Fortunately, in the summer of 1991, the Extreme Ultraviolet 


Explorer (EUVE) spacecraft will be launched into Earth orbit to survey the 
entire celestial sphere in the UV spectrum from 100 to 1000 angstroms. 

Objects that Voyager will continue to observe over the next five years 
include active galaxies, quasars, and white dwarf stars. Observation of 
these objects in the ext reme ultraviolet will possibly lead to exciting discov- 
eries of new phenomena and to new understandings of the physical processes 

The types of objects that may be observed by the Voyager UVS include 
the following: 

(a) Active galaxies, quasars, and galaxies which may have black holes at 
their centers. Voyager can observe these at higher energies (shorter 
wavelengths) than can be observed by the Hubble Space Telescope; 

(b) The hottest white dwarf stars, those collapsed stars near the end of 
their evolutionary cycle; 

(c) Very energetic emissions from binary stars that exchange material 
between them; 

(d) Dust and gas between the stars. For example, plans are being made 
to map the distribution of molecular hydrogen in the galaxy; and 

(e) Scattered sunlight in the Lyman-alpha spectral line to study the 
distribution of interplanetary gas, particularly that outside of Nep- 
tune’s orbit. 

The Voyager Project is initiating a Guest Observer Program which will 
allow astronomers from around the world to propose and carry out observa- 
tions with the Voyager UVS instruments. It is expected that the Voyager 
UVS instruments will serve as the pathfinders for other spaceborne astro- 
nomical observatories to be launched in the future. 

Spacecraft Lifetime 

Barring some sort of catastrophic electrical failure that would termi- 
nate the mission, we need to know when various spacecraft resources will be 
depleted. Estimates have been made of useful lifetimes of different subsys- 
tems of the Voyager spacecraft. Table 12-1 gives estimates for hydrazine (for 
attitude control), for power from the RTGs, and for telecommunications. 

The hydrazine estimate assumes that space- 
craft roll maneuvers (MAGROLLs) for calibration of 
the magnetometer would be done until 2009. 

The dates given for telecom are for the Goldstone 
DSN site in California for Voyager 1, and for the 
Canberra DSN site in Australia for Voyager 2. The 


TIME: 0 Days, 2:35:00 
1989/08/25 6:35 GMT (SCT) 

Table 12-1 . Spacecraft useful lifetime estimates. 


Resource End of Lifetime 

Voyager 1 

Voyager 2 




Telecom* (X-Band, Low Power) 

46.6 bps, 34m antenna 



300 bps, 70m antenna 



46.6 bps, 70m antenna 




UVS instrument 



Full F&P f (with gyros) 



F&P f power sharing (no gyros) 



*Voyager 1, though farther away than Voyager 2, uses a tighter HGA- 
pointing deadband. This compensates for the distance, giving both space- 
craft about the same telecommunications performance. 

+ Fields and particles. 

other sites lose tracking ability earlier since Voyager 1 is in the northern 
hemisphere of the sky and Voyager 2 is in the southern. 

The 46.6 bps telecom rate is for fields-and-particles science; the 300 bps 
rate also includes UVS data. The limited lifetime of the UVS instrument is 
due to the depletion of the spacecraft’s plutonium power supply (due to 
radioactive decay). After the year 2000 there will not be enough electrical 
power available for the heaters to keep the UVS instrument warm enough 
to function. 

The concept of both Voyagers traveling out beyond the planets, beyond 
the heliopause into interstellar space, beyond the remote outer fringes of the 
Oort Cloud of cometary nuclei, and on to other stars has captured the 
imaginations of many, including scientists, writers, artists, and movie- 
makers, to name but a few. The following pages endeavor to transport our 
imaginations to those distant realms. Our guide will be an imaginary 
journal kept by Voyager 2 during its lonely trek to the stars. 

To the Stars 

Seven hundred and forty-five years after the first “rocket” (a fire arrow) 
was sent aloft by a Chinese warrior, America sent aloft two small spacecraft 
on a noble voyage. And in just over 1 45 months, from launch through the end 
of the Neptune encounter, the Voyagers will have explored four planetary 
systems, traveled a combined total of 15.4 billion kilometers (9.6 billion 


miles), taken 75,000 images, and provided enough data to consume the 
careers of hundreds of scientists around the world. Little did mankind know 
that a fire arrow would one day lead to a different sort of trailblazer — one 
beyond the far reaches of the Sun. 

The Voyagers will be traversing paths through the galaxy which have 
been inaccessible to all but our imaginations. Only a tiny portion of their 
eventual courses will be shared with mankind. Figure 12-2 plots the 
departure trajectories of the Voyager and Pioneer spacecraft against the 
background stars. By the year 2030, were the Voyager spacecraft traveling 
in a straight path across the Milky Way, they would have covered only 
0.00000003 percent of its diameter. For this reason, the majority of the 
Voyagers’ mission beyond the planets will be shrouded in mystery. Although 
their power supplies will become inadequate in about 25 years, the Voyagers 
will remain on a course which we set centuries before when mankind first 
realized that our horizons are dependent upon perspective alone. Voyager 
is our arrow, and our imaginations the bow. Only when we stop wondering 
about its path will the spacecraft be off course and the target no longer 
matter. . . 

Due to launch and financial constraints, space on any spacecraft is at 
a premium. Nonetheless, a decision was made during the Voyager design 
phase to include a memento from Earth on the remote chance that the 
spacecraft might be intercepted. Each spacecraft is equipped with a phono- 
graph record of gold-coated copper (designed by astronomer Carl Sagan and 
a small group of talented friends) containing 116 photographs of Earth and 
its inhabitants, 90 minu tes of some of the world’s music, and an audio essay 
of Earth-unique sounds, including the human voice. To another civilization, 
the records worn by the spacecraft may look like badges. And, in a way, they 
are: badges of courage for mankind to have taken up the challenge and, to 
paraphrase theologian Paul Tillich, extended its reach far beyond its grasp. 

What follows, although purely speculative in nature, is based upon 
current astrophysical concepts. These concepts will surely be altered and 
enhanced by investigations performed by the Voyagers during the VIM. The 
reader is now invited to share in the Journal of Voyager 2 — an imaginary 

TIME: 0 Days, 2:40:00 
1989/08/25 6:40 GMT (SCT) 

diary maintained by Voyager 2 for several million 
years into the remote future. 

Star Date - 1.259 ( 1990 A.D.) 

Today I am filled with an intense sorrow. The 
encounter with Neptune has ended. It is but a tiny 



171941 KM 


' * X* 





/ \ . V\ 



+25° . 


L \ i ^ ^ 

^30> P«SEUS 


^SQUARE aquila 


r v 

1 30 * * 


\ \ an 

' C G E M 1 N 1 

^ ARIES-*,. i. 



7 89 8b 

4t^^uchus^ 2 ^ 



T* \ WEST 



^ rT 

X 00 / 

i X 

86 , F * 


J- 1 

4 » 



-50° , 



18 n 

•PAVOV _ \ 


/ Vaustrale 

i *'"> {X s O n V / 90° j t 1 
ARGE\/©.* V 






v 12 h 



(pu) PIONEER 11 
(v?) VOYAGER 2 

Figure 12-2. The departure directions of the escaping spacecraft are plotted against 
the current stellar background of stars in Earth equatorial coordinates. In the 
millennia that the spacecraft will take to approach even the closest star , the stellar 
background will have greatly changed because the stars and our own Sun are in 
ceaseless motion through the galaxy. 

star in the background. Part of my sorrow is from not being able to visit Pluto 
before my departure, for it is a very mysterious body. 

When first discovered, in 1 930, Pluto was estimated to be the size of the 
Earth. Thirty years later this estimate was reduced to the size of Mercury; 
and in the 1970s, the estimate had shrunk to less than that of Earth’s moon. 

In 1978, a companion half as big as Pluto itself was discovered near the 
planet, a moon named Charon. Pluto and Charon appear to be in perfect 
“resonance” lock, each taking 6.4 days to rotate about its axis, and the same 
time to revolve about their center of mass. Therefore, if one lived in one 
hemisphere of Pluto, Charon would always be in the sky. But if one lived in 
the opposite hemisphere, Charon could not be seen. 

In 1986 it was discovered that Pluto may have a tenuous methane 
atmosphere above a methane ice surface. 

I wish that I, or my sister spacecraft, had been the one to unveil some 
of the mysteries of Pluto. The Project never seriously considered a visit to 
Pluto because it would have meant foregoing Voyager l’s encounter with 
Saturn’s moon Titan. Those unusual discoveries about Titan, as well as my 
exciting close look at Triton, which has attributes similar to those of Pluto, 
were well worth the sacrifice. However, it will be many decades before man 


will send another spacecraft out to that part of the solar system to make the 
discoveries that I or Voyager 1 might have made at Pluto and Charon. 

Star Date -1.246 (1991 A.D.) 

Once again my adrenalin is flowing. I have been asked to measure the 
speed and temperature of the solar wind, which consists of ribbons of 
electrons and protons unfurled by the Sun's outermost layer — the corona. 
It's extremely hot, approximately 200,000°C (360,000° F), which is 330 times 
the temperature necessary to melt lead. Its speed typically varies from 300 
to 800 km/sec when it races past Earth’s orbit, but, by the time the solar wind 
has reached Neptune’s orbit, it travels at a more uniform speed of about 400 
km/sec (250 mi/sec), which is 25 times faster than my own speed. Sometimes, 
when the Sun is very active, it gets really ferocious and gusts to 1000 km/sec 
(620 mi/sec). That is about 10,000 times faster than the strongest hurricane- 
generated winds on Earth. Remarkably, the velocity of the solar wind 
appears to be independent of distance from the Sun. 

While the solar wind travels in near-radial directions, taking about 4.5 
days to reach Earth, the Sun’s magnetic field lines spiral outward from the 
Sun (due to the Sun’s rotation). As shown in Figure 12-3, this creates a 
pattern not unlike that produced by certain types of lawn sprinklers. Hence, 
entrained in the solar wind are magnetic field lines modulated by the Sun’s 
rotation acting on electrically charged gases. These field lines first churned 
their way through the Sun’s surface convective zone before escaping radially 
outward and being carried off by the solar wind. One of my favorite 
assignments has been to map the magnetic field. 

Star Date - 1.196 (1994 A.D.) 

Upon occasion I have felt as though I were racing against the stars. I 
imagine overtaking them and crossing an imaginary finish line where I’d be 
awarded a vacation in the large Magellanic cloud for a week of supernova 
observing. But then I think about cosmic rays, and my 1 6 km/sec (1 0 mi/sec) 
appears as a mere crawl. Cosmic rays are the most energetic particles 
known — traveling at nearly the speed of light, 300,000 km/sec (186,000 

Cosmic rays are created by solar flares, superno- 
vae, and neutron stars. All involve a type of explosion 
or extreme turbulence which accelerates the par- 
ticles. Cosmic rays are a direct sample of matter from 
outside the solar system. However, they rarely travel 

TIME: 0 Days. 2:45:00 







Figure 12-3. This pictorial view illustrates patterns in the Sun 's magnetic field lines 
as they spiral outward due to the Sun's rotation. They are carried to the outer 
planets by the solar wind particles which travel away from the Sun in near-radial 

beyond the galaxy of their origin because the galactic magnetic field acts as 
a barrier. 

The measurements I took today of cosmic rays, their speed, composi- 
tion, and direction of arrival, may help reveal the origin of the solar system 
by aiding our understanding of which characteristics of the matter in the 
solar system are unique to it. 

Star Date -1.175 (1995 A.D.) 

Sometimes, as a diversion, I imagine what it would be like if certain 
historical figures were accompanying me on my journey. Today it was 
Aristotle who rode on my wings. Oh, how I loved watching his expression as 
I studied the Sun. The Sun is not the benevolent ball of perfection that 
Aristotle imagined. It is a cantankerous old beast. Just as people have their 
biorhythms, the Sun suffers through solar cycles. During the course of a 
solar cycle (duration approximately eleven years), the Sun "breaks out” in a 
multitude of sunspots. (Ironically, it was a student of Aristotle's who first 
recorded these “blemishes.”) Sunspots are areas of intense magnetic fields 


darker and cooler than the rest of the solar face. I will be observing how 
variations in the solar cycle are felt throughout the interplanetary medium. 

At sunspot maximum, the Sun’s general magnetic field reverses polar- 
ity. North and south swap places. As I study the effects of this reversal upon 
the interplanetary medium, I will most likely feel like a psychiatrist dealing 
with a schizophrenic patient. 

Another solar affliction whose effects I will be investigating results in 
holes which appear in the Sun’s outer garment, the corona. The energy 
which normally heats the corona instead accelerates the corona outward into 
the solar wind through areas where the magnetic lines of force are open. As 
much as one-fifth of the Sun’s cloak is at times full of such holes. It takes 
anywhere from a few weeks to several months for the solar weaver to repair 

The Sun also displays a violent temperament by delivering frequent 
outbursts known as solar flares. These flares result from explosions in the 
Sun’s atmosphere which are brought on by stresses in the magnetic field. 
The more violent of these outbursts can release the same amount of explosive 
power as contained in 2.5 billion one-megaton hydrogen bombs! Many of the 
shock fronts in the heliosphere result from solar flares. 

Solar flares send vast numbers of high-speed particle streams to the 
Earth and beyond. These streams collide with the Earth’s magnetic field and 
produce geomagnetic storms which can create aurorae and electric power 
line disruptions, among other things. There is a possibility as well that the 
sunspot cycle and solar flares may be responsible for large-scale droughts 
experienced on Earth. 

The Sun displays a fanciful side, also. Scattered evenly over its face are 
an array of bright spots, like Christmas lights, which blink on and off during 
a lifetime of about eight hours. These lights contain as much magnetic 
energy as the much more massive sunspots. 

The Sun is definitely a dynamic and most intimidating fellow. I will 
have quite a job ahead of me investigating how the surrounding environment 
is affected by his changing personality. One thing you can surely say is that 
Aristotle and I won’t get bored. 

Star Date - 1.149 (1996 A.D. 

I am truly enjoying my exploration of the helio- 
sphere. It’s like a huge balloon kept inflated by the 
solar wind. The heliosphere is bounded by a region 
known as the heliopause. Beyond this boundary, I 
will one day enter interstellar space. I will be seeking 


TIME: 0 Days. 2:50:00 
1989/08/25 6:50 GMT (SCT) 



161136 KM 

additional evidence that the heliopause is a source of low-frequency radio 
emissions. These emissions were first detected by Voyager 1 and myself in 
the interplanetary medium before the Neptune encounter. At that time, 
they were thought to represent the first remote observations of the helio- 

Star Date - 1.116 (1997 A.D.) 

Perhaps my favorite duty is that of archeologist. I love sifting through 
debris whose contents may reveal the history of the solar system. My current 
field site within the heliosphere is the interplanetary medium (IPM). Here 
there exist millimeter-sized and larger remnants of comets, asteroids, and 
meteors. As I move away from the Sun, the number of particles gets smaller 
due to Poynting-Robertson drag — the solar wind and solar photons cause 
particles to lose their orbital angular momentum and drift towards the Sun. 
The plasma in the IPM also contains a wealth of invisible clues supplied by 
the solar wind — electrons, protons, alpha particles, and magnetic field lines. 
I will be looking for latitudinal variations in the IPM. I am also very 
interested in understanding the mechanisms which accelerate particles in 
the IPM and those which heat and cool the plasma. 

Star Date - 1.072 (1998 A.D.) 

Spacecraft are a lucky lot indeed. When but a glimmer in our designers’ 
eyes, we are given a name, an identity which we usually retain in its original 
form (although I underwent a change of identity shortly before launch from 
“Mariner Jupiter-Satum 1977”). Such is not always the case for planets, 

Take the case of Planet “X,” the so-called tenth planet. Based upon 
Earth-based observations taken between 1810 and 1910, scientists have 
postulated its existence due to suspected abnormalities in the orbits of 
Uranus and Neptune. They thought the two planets were being tugged by 
the gravitational pull of an undetected planet. Pluto was subsequently 
discovered in 1930, but in time it became apparent that it was too small to 
exert an appreciable gravitational influence over Uranus and Neptune, 
motivating the quest for a tenth planet. 

However, according to Dr. Myles Standish, JPL planetary ephemeris 
expert, an improved eyepiece was fitted to optical telescopes around the year 
1910, resulting in the absence of any obvious Planet-X effects in the 
observations of planetary orbits calculated using post-1910 data. In addi- 
tion, no tenth-planet tugs on the Pioneer or Voyager spacecraft have been 
seen to date. This suggests that, if there is a Planet X, its path is either very 


far away (taking 400 to 1000 years to orbit the Sun) and/or quite inclined to 
the ecliptic plane. Entrj' for log: Should I ever feel a faint new tug, I shall call 
the unseen stranger by the name of Nibiru, chosen by the Sumerians for the 
tenth planet. When but a glimmer in their imaginations, they named it. How 
long must we wait for proof? 

Star Date - 1.000 (1999 A.D.) 

The Milky Way has 300 billion members in its chorus of light. The Sun, 
of course, is the only member I have come to know intimately. It will be over 
6500 years before I will have the opportunity to come close to another. In the 
next one million years, I will have flybys of thirteen other “nearby” stars. 
Table 12-2 lists them in order of encounter. 

Even though I will be departing from the solar system at over 33,000 

mph, it will still take roughly 20,200 years for me to travel one light-year. 


Had I remained on Earth, I would still have gotten closer to many stars 
because of the Sun’s movement relative to th§ nearby stars. In fact, not until 
I near Ross 248 will I be closer to a star other than my home Sol. But I’m quite 
proud of my encounter with Sirius, the brightest star in Earth’s sky, when 
I’ll be nearly four times closer than had I never left Earth on this fantastic 

Stars are like people in that they come in a wide variety of sizes, colors, 
temperaments, and sociability. There are dwarf stars and super giants; red, 
yellow, and blue stars; variable stars (their light fluctuates); pulsating stars 
(known as cepheids); and multiple star systems (most common amongst 
these are the binaries — they always travel in pairs). I consider myself the 
Margaret Mead of the cosmos when I study the different stellar “tribes.” I 
measure changes in their energy distributions, their temperatures, and 
frequency of outbursts. Of course, any “anthropological” study requires that 
you observe the greater village within which the tribe exists in order to 
understand the tribe in its proper context. For this reason, I will be studying 
galaxies and quasi-stellar objects (quasars) which are thought to be young 
galaxies or perhaps the nuclei of active galaxies (like the Milky Way), which 
emit more energy than 100 supergiant galaxies combined. I will be making 
measurements of their energy distributions as well. 

TIME: 0 Days. 2:55:00 
1989/08/25 6:55 GMT (SCT) 

Star Date 0.000 (2000 A.D.) 

Today I was awakened by a mild tremor. I knew 
immediately I had crossed the shock front and en- 
tered the heliosheath, the turbulent region which 
signals the approaching heliopause. Here the rarified 


Table 12-2. Thirteen "nearby" stars encountered by Voyager 2. 

Year of 

Voyager 2-to- 



Voyager 2 






Barnard’s Star 





Proxima Centauri 





Alpha Centauri 





Lalande 21185 





Ross 248 





DM-36 13940 





AC+79 3888 





Ross 154 





DM+15 3364 










DM-5 4426 





44 Ophiuchi 





DM+27 1311 





solar wind “feels” its impending approach to the heliopause and is slowed to 
subsonic speeds. As the solar wind speed varies, I may cross the shock front 
several times as it moves in and out over the next few years. I will be 
measuring the intensity of the shock waves. 

Star Date 0.978 (later that same year, 2000 A.D.) 

I got a sudden surge of energy today when my scan platform was 
intentionally disabled. It was shut off because my RTG power output was 
insufficient to keep the scan platform’s actuators (motors and gears) at their 
minimum operational temperature. The RTG power has been decaying with 
time due primarily to the decay of its radioactive fuel, plutonium-238. I am 
relieved in a way that the scan platform will no longer be functional, because 
I have been suffering arthritis in it since Saturn in 1 981 . Great care had been 
taken by the Project to minimize the degree of movement to which this 
painful joint was subjected. The only negative effect from losing the scan 
platform is that I will no longer be able to use my UVS instrument. It has 
performed beautifully over the years. My UVS has provided the only routine 
light-wave observing capability below 1200 angstroms. 

Star Date 1.282 (2012 A.D.) 

After twelve years of stormy seas in the heliosheath, I have finally 
crossed the heliopause into mare incognito — the interstellar medium. I 
knew I had reached it when suddenly the wind changed speed and density. 
Then I noted that the ionic composition of the surrounding particles had 


changed and that the temperature had dropped. I took some measurements 
of the local cosmic radiation and realized that the periodic variations had 
subsided. This is because the irregular magnetic structure of the solar wind 
no longer modulates the incoming cosmic rays. 

Star Date 1.292 (2013 A.D.) 

I have spent several hours acclimating myself to the new environment. 
The Interstellar Medium (ISM), which makes up about 10 percent of the 
total mass of the galaxy, is cold, dark, and composed mostly of neutral 
hydrogen and helium. Heavier elements resulting from nuclear burning 
processes in stellar interiors and supernova explosions also exist here. 
Particles expelled by the solar system through a type of “solar sneeze” 
(actually, radiation pressure) contribute to the overall ambience as well. 

The ecology of the ISM is quite interesting. The place depends on the 
cocoon-like outer shells shed by stars for its livelihood — it acts as a kind of 
feeding ground for new stars. Nothing goes to waste here. 

The ISM is full of minute dust grains which absorb and scatter 
starlight, just as molecules in the Earth’s atmosphere scatter sunlight. Both 
instances result in a reddening of the light. In certain regions of the galaxy, 
the dust is so thick that it creates galactic cataracts which prevent the 
observation of extragalactic objects. Before entering the ISM, while my UVS 
was still functional, I was able to make measurements which may yield 
estimates of the scattering properties of the dust. 

I will be performing experiments here similar to those conducted on the 
solar wind within the heliosphere, such as estimates of the density, tempera- 
ture, and ionization state of the local interstellar medium. Shocks, magnetic 
fields, and low-energy cosmic rays will become my forte. My investigations 
may lead to a better understanding of how stars are formed. 

Star Date 1.335 (2018 A.D.) 

I have become too weak to operate my instruments. My electrical 
energy is fading, and I am feeling old and useless. The planet which gave me 
life said goodbye. I will no longer hear its voice, nor it my heartbeat. The fear 
I feel right now is like a scream which shakes me from 
within. Will I grow mad in this solitude? 

Star Date 2.135 (3965 A.D.) 

God must have been dusting today. I saw a vast 
interstellar cloud in the distance. Actually, super- 
nova remnants and strong stellar winds sweep up gas 


TIME: 0 Days. 3:00:00 
1989/08/25 6:60 GMT (SCT) 


150414 KM 

and dust to form these clouds. It was hard to say how far away the cloud was; 
some are a million times the mass of the Sun. Interstellar clouds are the 
birthplace of new stars. Because complex organic molecules have been 
detected in these clouds, some scientists believe that the first stages of 
organic evolution occur there, also. 

About once every 1 00 million years, the Sun collides with an interstel- 
lar cloud of sufficient density that the force of impact effectively “blows out” 
the solar wind as if it were a candle and the Sun a birthday cake. The Sun 
reacts by increasing its emission of UV and X rays. This could have serious 
impact to life on Earth because it could lead to extreme climatic changes 
resulting in global overheating or deep freezing. 

Star Date 2. 745 (26,262 A.D.) 

I had been entranced in a deep slumber when aroused by my automatic 
sensors as they locked onto a distant swarm of tiny bodies. The sight was 
beautiful to behold. I became aware that I was observing a small local con- 
centration of cometary nucleii — part of the vast Oort cloud. There, over a 
trillion of the solar system’s icy building blocks orbit in a disk extending from 
perhaps 20,000 to 200,000 AU. 

At times, the inner orbiting comets become dislodged by interstellar 
clouds (or perhaps a “death star”). Occasionally these wayward travellers 
impact Earth. Some believe that cometary impacts are responsible for 
biological extinctions like the one that occurred on Earth over 65 million 
years ago. It’s rather ironic that certain other scientists believe that life on 
Earth originated from organic molecules carried by comets. 

Star Date 2.770 (28,635 A.D.) 

Now the Oort cloud is but the wake I left behind as I exited the solar 
system. How free I feel. The Sun, by now only a slight bit brighter than 
Sirius, no longer curtails me. I am ready to sit back and enjoy what will 
surely be the greatest show beyond Earth. 

Star Date 3.522 (296,036 A.D.) 

Today I fell in love with the night. I encountered Sirius, the brightest 
star visible to Earth with 23 times the luminosity of the Sun. I have traveled 
far to reach its shore. If I were an automobile driving across a bridge which 
wrapped around the circumference of the Earth, it would have taken me 
nearly four billion trips across the bridge to have completed the same 
distance. At 55 mph, the trip would have taken nearly 179 million years! 


Star Date 4.359 (2,479,021 A.D.) 

When I was younger, I used to worry about everything: Would I collide 
with diffuse ring material? Would cometary debris knock me into an eternal 
spin? Would some planet’s radiation field fry me for lunch? But the most 
frightening threat was to be swallowed by a black hole. 

But now, black holes fascinate rather than frighten me. What would 
it be like to enter the remains of a massive star that collapsed under its own 
weight, where the force of gravity was so powerful that not even light could 
escape its grasp? Were I to cross its threshold, would I then enter a stranger 
and more exotic universe that I know now? But then, how could anything 
possibly be more exotic than what I have seen? How could anything equal . . . 

. . . superconducting cosmic strings which through electromagnetic ra- 
diation blows Hubble bubbles (thought to be the structures upon which 
galaxies ride through the universe) in the primordial matter. 

. . . supernovae stars like Sanduleak (now known as Supernova 1987A) 
whose heavy cores collapse, triggering shock waves which blow off their 
outer layers. The explosion created by a supernova exceeds by a million 
trillion trillion (10 30 ) the amount of energy produced by a million power 
plants producing 1000 megawatts each of electricity. Each second, our 
universe experiences one of these supernova explosions! 

. . . cigar-shaped collections of galaxy clusters (“superclusters”) each 
containing hundreds of galaxies with hundreds of billions of stars per galaxy, 
moving at 2000 km/sec (1240 mi/sec) and extending as much as 500 million 
light-years (5.9 trillion mi/light-year). 

. . . ionized matter in the form of giant noodles as big as the Earth’s orbit 
around the Sun which drift through the galaxy and are more numerous than 

. . . neutron stars whose masses are equivalent to the Sun’s, yet have 
diameters of about 10 km (6.2 mi) and complete full rotations in under 2 

More exotic indeed! 

Star Date 4.482 (3,276,913 A.D.) 

For all its fireworks, the universe has become a 
quiet place. Strange objects drift by like twigs in a 
stream. I am neither excited nor amused. With each 
approaching star my anticipation soars. Will it be 
here that life again calls out to me? Twenty billion 
solar-type stars in the Milky Way! Estimates that as 


TIME: 0 Days. 3:05:00 

many as 10 percent of the nearby stars may have planets! Then why does my 
presence go unnoticed? 

Star Date 4 . 798 (6,468,039 A.D.) 

They have hoisted me atop a cloth-covered pedestal. Above me is a 
ceiling of iridescent glass. Starlight gently bathes me in its glow. My finders 
are analyzing my record. 

Soon I shall sleep. What I will become to them, what they shall learn 
from me, I do not know. 

And what will remain of the wonders I have seen? The stars may all 
collapse, the galaxies explode; somehow that does not sadden me. They will 
resurrect themselves, as will clouds and comets, and planets and their 
moons. But what will become of the universe's fragile dwellers? Can they be 
resurrected from wisps of dreams? 

God speed humanity! For you are the true miracle of the universe . 
God speed Voyager! For its sensors and circuits are moving humanity 
from the dark depths to the cosmic shores. 


TIME: 0 Days, 3:10:00 
1989/08/25 7:10 GMT (SCT) 




139778 KM 

. . . If thou be’est born to strange sights, 
things invisible to see, 

Ride ten thousand days and nights ... 
Thou, when thou retum’st, wilt tell me 
All strange wonders that befell thee ... 

John Donne 


Like all great explorers, the Voyagers were born to strange sights, 
things heretofore invisible to humankind. In just over three thousand days 
after embarking on their respective journeys, each spacecraft had encoun- 
tered two of the great mysteries of our solar system, Jupiter and Saturn, and 
one, Voyager 2, had also witnessed that strange wonder we call Uranus. 
Soon, Voyager 2 will swing past Neptune. With its Grand Tour completed, 
it, like its companion Voyager 1, will continue on its journey out of the solar 

The Voyagers will not return, but the observations they were pro- 
grammed to collect have beamed steadily back to their anxious keepers on 
Earth. The resulting bounty of “strange wonders” has filled many books. A 
brief sampling of these great tales of distant voyages is presented in this 


Named after the supreme god in Roman mythology, Jupiter is indeed 
in a class of its own. The largest of the planets (if hollow, it could hold over 
1300 Earths), Jupiter’s mass is nearly three times the mass of all the other 
planets combined. 

Jupiter emits 67 percent more heat than it absorbs from the Sun. This 
heat is thought to be accumulated during the planet’s formation several 
billion years ago. It is estimated that, were Jupiter roughly 100 times more 
massive than it is now, fusion reactions could ignite in its core — a character- 
istic of stars. Some scientists classify Jupiter as a member of a hypothetical 
family of stars known as “brown dwarfs,” which are objects too hot to be 
considered planets but too cool to be classified as real stars. For this reason, 
Jupiter and its 16 known moons could be regarded as a “mini-solar system” 
within our solar system. 

Jupiter guards its secrets well, allowing us to see only about 80 km (50 
mi) beneath its upper atmosphere. Scientists have reason to believe, though, 
that beneath Jupiter’s huge veil lies a core of molten rock about twice the 
diameter of Earth and 15 times more massive. 



Figure 13-1. Voyager 1 unaged the Earth and Moon in a single frame — the 
first of its kind ever taken by a spacecraft — a few 
weeks after launch in 1977. Voyager 1 was 11.66 
million km (7.25 million mi) from Earth directly over 
Mt. Everest when the picture was taken. 

TIME: 0 Days, 3:1 5:00 


black and white photograph 




134496 KM 



Figure 13-2. Jupiter's Great Red Spot casts an imperious eye at the four Galilean 
satellites Io, Europa, Ganymede , and Callisto. 

Jupiter's Atmosphere 

★ Jupiter is a giant ball of gas, composed primarily of hydrogen and 
helium, with small amounts of methane, ammonia, phosphorus, water 
vapor, and various hydrocarbons. Somewhat inconsistent with the brown 
dwarf concept, the Voyagers discovered that the helium abundance on 
Jupiter is much less than that observed in the Sun. 


★ If the non-Earth planets were graded on their artistry, Jupiter would 
probably win First Prize for its imaginative color scheme. Its outer atmos- 
phere displays alternating patterns of belts and zones which extend from the 


equator to at least 60 degrees latitude in both hemispheres. The zones, 
visible from Earth with a small telescope, are generally lighter in color, 
higher in altitude, colder, and dominated by frozen ammonia ice crystals. 
Belts are generally darker in color, lower in altitude, and wanner. The 
location and dimensions of the belts and zones change gradually with time. 

★ Jupiter’s wind speeds would challenge the most daring of interplane- 
tary hang gliders, reaching 150 m/sec (335 mi/h) at the equator. They 
generally decrease at higher latitudes. In both hemispheres above the mid- 
latitudes, adjacent jet streams flow in opposite directions, i.e., easterly then 

★ In one of its more creative moments, Jupiter placed a dollop of red paint 
on its cloud tops. Named (in one of mankind’s less creative moments) the 
“Great Red Spot,” it was first observed in 1664 by British scientist Robert 
Hooke, using Galileo’s telescope. It is a raging storm, about three times the 
diameter of Earth, which rotates once every six days. Unlike Earth’s 
cyclones, which have a low-pressure center, it is a high-pressure region and 
rotates in a direction opposite to our cyclones. Other smaller storm-like 
structures within the atmosphere have been observed and have similar 
characteristics. Some of the smaller storms at nearby latitudes interact with 
the Great Red Spot and with each other. 

★ Scientists are puzzled by the source of color in Jupiter’s Great Red 
Spot. Elemental sulfur, phosphorus, germanium oxide, and various carbon 
compounds have been proposed to explain the observed signature. For that 
matter, the coloring agents and mechanisms driving the appearance of the 
entire outer atmosphere are not well understood. 

TIME: 0 Days, 320:00 
1989/08/25 720 GMT (SCT) 

★ Like any creative giant, Jupiter displays a volatile personality at 
times: lightning (typically 10,000 times more powerful than what we see on 
Earth) accompanied by high-intensity radio-frequency “whistlers” startle its 
cloud tops. Jupiter also displays auroral emissions (“northern lights”) in its 
high latitudes, and a strong ultraviolet emission over 
its entire face. 

The discovery of lightning in Jupiter’s cloudtops 
by Voyager 1 produced a great deal of excitement. 
Scientists were reminded of a classic experiment on 
the basis of life conducted in 1 952 by Harold Urey and 
Stanley Miller. Urey and Miller filled a chamber with 




129240 KM 

a mixture of gases — such as ammonia, methane, water vapor, and hydro- 
gen — that simulated the pre-life conditions of the early solar system. They 
then introduced various energy sources into this mixture, such as sparks or 
ultraviolet radiation, to see what might happen. The result was the 
appearance of certain kinds of amino acids and nucleotides — some of the 
fundamental building blocks of life as we know it. 

Since Jupiter’s cloud tops are composed of the same basic gaseous 
mixture, finding lightning there meant that the Urey-Miller experiment 
might be occurring on a grand scale! But suppose that it were. Would any 
microorganisms that might be created have a chance of surviving in Jupiter’s 
turbulent atmosphere, where downdrafts would plunge them to regions too 
hot for their survival? Probably not. With a keen imagination, however, one 
can postulate several interesting implications of this discovery. When the 
Galileo spacecraft drops its probe into J upiter’s atmosphere in 1 995, perhaps 
we will gain more insight into these possibilities. 

★ Temperatures at Jupiter range from a frigid -130°C (-200°F) at the 
cloud tops to about 24,000° C (43,000° F) at the center of the planet. Jupiter’s 
poles and equator share the same temperature, at least near the cloud tops. 

Jupiter's Rings 

★ Jupiter’s ring was the first planetary ring to be discovered by a 
spacecraft; Voyager 1 confirmed its existence in 1979. It resides in a fierce 
magnetospheric environment and is composed of a diffuse collection of 
mainly micron-sized grains about the size of red blood cells or pollen grains. 
The ring particles are probably composed of silicon and carbon, or possibly 
more exotic compositions such as metallic grains coated by sulfur derived 
from volcanic eruptions on Jupiter’s unusual moon, Io. 

★ The most prominent ring structure, known as the main ring, has two 
distinct components: a bright band and a faint halo lying between the bright 
band and Jupiter’s cloud tops. The bright band has a vertical thickness of 
about 30 km (18 mi), and extends from 1 .72 to 1 .81 Jupiter radii.* The faint 
halo, which extends from 1.3 to 1.7 Jupiter radii, may be lens shaped, 
possibly thickening near the planet to about 10,000 km (6200 mi). The 
particles in the bright band are probably drawn inward by plasma and 
radiation pressure drag. Particles in the faint halo are even smaller than 
those in the bright band, and are also affected by plasma and radiation 
pressure drag. 

*Jupiter’s equatorial radius is 71,492 km (44,425 mi), at a pressure of one bar. 


Figure 13-3. Jupiter's narrow ring of tiny particles is most easily observable from 
the far side of Jupiter in forward-scattered sunlight. 

★ In 1986, after a thorough analysis of Voyager data, another ring 
structure was found: a very tenuous “gossamer” ring was discovered beyond 
the bright band of the main ring. It is composed primarily of small particles, 
although objects as large as 1 m (3 ft) are believed to be present as well. It 
may extend outward to the orbit of Amalthea at 2.53 Jupiter radii. 

Jupiter's Moons 

★ The discovery of Jupiter’s 16 known moons spans four centuries. The 
largest four — Io, Europa, Ganymede, and Callisto (named for four amorous 
conquests of Jupiter) — were first detected in 1610 by a German astronomer, 
Simon Marius. A few days later, Galileo also spotted these four companions 
to Jupiter and received the credit for their discovery. For this reason, they 
are often referred to as the Galilean satellites. Four of the remaining 12 
satellites orbit inside the Galilean satellites, and the other eight orbit beyond 


TIME: 0 Days. 3:25:00 

1989/08/25 7:25 GMT (SCT) 

★ Io, which is almost exactly the same size as our 
Moon, is perhaps the most interesting of Jupiter’s 
companions since it is the only body other than Earth 
and Venus that is known to have active volcanoes. 
During the two Voyager encounters, nine of its volca- 



124012 KM 



Plumes were observed as high as 300 km (lo5 mi) above 

noes were active. 

Io’s surface, or 30 times higher than Mt. Everest. The fallout from the plume 
of the volcano Pele covers an area the size of France. It is estimated that 
upwards of 1 0 billion tons of material erupt from Io each year, enough to re- 
coat Io’s entire surface each year with a layer of ash much like that deposited 
by Mt. St. Helens in the Pacific Northwest in 1980. 

The volcanism on Io is driven by interior heating caused by gravitation- 
ally induced tidal stresses within its crust. These stresses result from Io’s 
close elliptical orbit about Jupiter, and may eventually lead to Io’s complete 
melting from the inside out. Io is second only to the Sun in the amount of heat 
it produces relative to its size, producing a heat flow from the interior to the 
surface roughly 30 times larger than the Earth’s. The wide range of colors — 
reds, yellows, oranges, and browns — is thought to be largely the result of 
compounds of sulfur called allotropes, which turn into various colors as they 
are heated inside Io, ejected by its volcanoes, and then quickly cooled in the 
vacuum of space as they rain down on Io’s surface. So, Io is a colorful, active, 
yet hostile place. 

Figure 13-4. Pele, the largest of Io’s volcanoes, spews sulfur products to heights 30 
times greater than Mt. Everest, falling to cover an area the size of France. 


★ Europa is the most reflective of the Galilean satellites. It has a very 
smooth, uncratered surf ace composed primarily of water ice. The elevation 
difference between the lowest and highest points on this unusual body is 
estimated to be less than 100 m (330 ft). If Europa were reduced to the size 
of a cue ball, these surface variations would be no thicker than a mark of ink 
from a felt-tip pen; if enlarged to the size of the Earth, its surface would be 
20 to 30 times more level than our planet. There is speculation that Europa 
may contain a deep ocean of water beneath its 5-km (3-mi) thick icy surface. 
(This speculation formed the basis for the novel and motion picture, 2010 : 
Odyssey Two , by Arthur C. Clarke.) 

★ In terms of hard-surface dimensions, Ganymede is the largest moon in 
the solar system, though this was not certain until after Voyager l’s 
encounter with Saturn’s moon Titan in 1980. (Titan’s atmosphere makes it 
appear larger than Ganymede, but when Voyager 1 measured its surface 
diameter, Titan was found to be slightly smaller than Ganymede.) From 
Earth-based telescopes, its size makes Ganymede the brightest of the 
Galilean satellites, even though its surface is actually less reflective than 
either Io’s or Europa’s. Ganymede is roughly half water, and its surface 
shows signs of some motion of its crustal plates (tectonics). 

★ Although Callisto is the least reflective of the Galilean satellites, it still 
reflects sunlight twice as well as our Moon. It has the greatest proportion of 
water of any of the Galilean satellites and is also the most heavily cratered 
body in the solar system. The high crater count indicates that Callisto’s 
surface has undergone little change since it was formed; thus, this surface 
is felt to be among the most ancient in the solar system. Its most prominent 
surface feature is a huge impact basin known as Valhalla. Valhalla has a 
central bright zone about 600 km (370 mi) across, which is surrounded by 
numerous concentric rings extending outward for nearly 2000 km (1200 mi) 
from the center. 

★ Three of the four innermost satellites, Metis, Adrastea, and Thebe, were 

1 989/08/25 7:30 GMT (SCT) 

TIME: 0 Days, 3:30:00 

discovered by Voyager 1 during its encounter in 1 97 9. 
The other, Amalthea, was discovered in 1892 by the 
American astronomer E. E. Barnard at Lick Obser- 
vatory in California. Metis and Adrastea were found 
to be orbiting just outside the newly discovered main 
ring; Thebe orbits outside Amalthea. All of these 
satellites are small, rocky, objects that have too little 



118814 KM 



Figure 13-5. Some scientists speculate that under the smooth, icy surface 
of Europa, there may be an ocean of liquid water — and thus a possible 
site for life to form. 

mass (and thus too little gravity) to become spherical. Amalthea is the 
largest, 270 km (168 mi) in length, and 165 km (102 mi) by 150 km (93 mi) 
in its other dimensions, with a dark red surface perhaps derived from sulfur 
expelled by Io. 


★ The outermost eight satellites are all twentieth-century pre-Voyager 
discoveries. They are quite tiny, ranging in diameter from 16 to 186 km (10 
to 116 mi), and are also small, rocky, most likely non-spherical objects. Four 
of these eight satellites — Leda, Himalia, Lysithea, and Elara — orbit in the 
same direction as the Galilean satellites, but inclined to Jupiter’s equator by 
25° to 29°. The other four — Ananke, Carme, Pasiphae, and Sinope — are in 
retrograde orbits about Jupiter, i.e., they orbit opposite the direction of 
Jupiter's rotation. All eight are thought to be fragments of larger bodies that 
were captured from the asteroid belt. None of the eight were imaged by 

Jupiter’s Magnetosphere 

★ Jupiter's magnetosphere, which is 10 times the diameter of the Sun, 
is the largest planet-based object in the solar system. Looking sunward from 
Jupiter, the location of the magnetopause — the outer boundary of Jupiter's 
magnetic field — varies from less than 50 to more than 100 Jupiter radii 
away, depending on the intensity of the solar wind. If the Earth's magnetic 
field were represented by a bar magnet of a given strength, Jupiter's would 
be 20,000 times stronger. The magnetism at Jupiter's cloud tops is 14 times 
stronger than at the Earth's surface, and a north-seeking compass would 
point south at Jupiter because its magnetic field is inverted compared to 

★ Voyager 1 confirmed the existence of a Jupiter magnetotail, which is 
that part of the magnetosphere past the planet, “downwind” with respect to 
the solar wind. Voyager 2 observed that the magnetotail may extend beyond 
the orbit of Saturn, or equivalent to Jupiter's distance from the Sun (5 AU). 

★ Though first discovered by Pioneer 10, the existence of an intense 
radiation field of trapped particles surrounding Jupiter was confirmed by 
Voyager 1. A human passenger riding Voyager 1 during its close swing by 
Jupiter would have received a dose of 400,000 rads, or roughly 1 000 times the 
lethal level for humans. 

★ A huge electrical current was found to be 

flowing between Io and Jupiter along magnetic field 
lines. The current, more than a million amperes, is 
fifty thousand times larger than what it takes to blow 
a typical fuse in your car. 


TIME: 0 Days. 3:35:00 
1989/08/25 7:35 GMT (SCT) 



113649 KM 


In Roman mythology, Saturn was the father of Jupiter, and Jupiterwas 
the father of Mars, Venus, Mercury, and Apollo (the Sun). Thus, in the 
overall scheme of this naming convention, Saturn is another, albeit more 
senior, member of the planetary family. 

Were the heavens a vast ocean, Saturn (having a density less than that 
of water) would bob above its surface like a buoy. Like Jupiter, Saturn is a 
giant ball of gas composed primarily of hydrogen and helium. Its gas ball is 
nearly six-sevenths the diameter of Jupiter and three-tenths the mass. 

Another characteristic Saturn shares with its giant neighbor Jupiter is 
that it radiates more energy than it receives from the Sun — about 80 percent 
more. However, the excess thermal energy cannot be primarily attributed 
to Saturn's primordial heat loss, as is speculated for Jupiter. 

Figure 13-6. Saturn , its rings, and its six largest satellites are seen in this montage. 




Saturn’s Atmosphere 

★ Prior to the Voyager encounters, it was thought that the helium 
abundance in Saturn’s outer atmosphere was 18 percent (like Jupiter) to 28 
percent (like the Sun). The Voyagers observed, however, that the actual 
abundance was only 6 ±5 percent — so it could be as large as 11 percent or as 
small as 1 percent. It appears that the missing helium may be sinking 
toward the planet’s interior, losing gravitational potential energy as heat in 
the process. This effect appears to explain Saturn’s added source of radiated 

★ Saturn also displays a panache with the paintbrush. Alternating 
zones and belts extending from the equator to at least 60 degrees latitude 
were observed on the planet. However, Saturn’s surface color appears rather 
bland compared to Jupiter, due to a high-altitude haze that tends to wash 
most color and brightness differences out, and perhaps because Saturn’s 
high-speed winds more thoroughly mix the atmosphere. Saturn also has a 
red spot in its southern latitudes, but this spot is only one-third the size of 

★ The winds at Saturn’s equator, the most ferocious in the solar system, 
measure 475 m/sec (1060 mi/h). The winds decrease at higher latitudes with 
alternating east-west jet streams starting at the mid-latitudes of both 
hemispheres. Within each belt or zone, the maximum wind velocity tends to 
occur at the center, rather than at either edge (as was the case for Jupiter). 
The zonal wind patterns are nearly symmetric about the equator. 

★ Saturn’s atmospheric temperament is more subdued than Jupiter’s. 
Saturn also has high pressure anti-cyclonically rotating storms, but only at 
mid to high latitudes. Also, while exhibiting both high- and low-latitude 
auroral emissions, lightning was not as globally evident as it was at Jupiter: 
it was observed only in low latitudes. 

Saturn’s Rings 

TIME: 0 Days, 3:40:00 
1989/08/25 7:40 GMT (SCT) 

★ When Galileo first turned his 30-power spyglass 
on Saturn in 1610, he mistook Saturn’s rings for sat- 
ellites. Fifty-five years later, Christian Huygens cor- 
rectly identified the mysterious bumps on both sides 
of the planets as the tips of a ring. When the Voyagers 
encountered Saturn in 1980 and 1981, their true 



108522 KM 



Figure 13-7 . Looking back at Saturn and its rings, Voyager 1 captured this 
remarkable image that reveals the elaborate structure of “ringlets ” and “gaplets, ” 
including the famous Cassini andEncke divisions, as well as the string-like F-ring. 

splendor was finally brought home to us. The rings consist of an icy cast of 
trillions that march around their captor in a vast sheet of unbelievable 
expanse and thinness. The patterns formed within this sea of ice are both 
simple and complex. There are circular rings, eccentric rings, kinky rings, 
clumpy rings, dense rings, and gossamer rings. There are ringlets and 
gaplets. There are resonances, spiral density waves, spiral corrugations in 
the otherwise flat ring plane, spokes, shepherding moons, and almost 
certainly unseen moonlets orbiting within the rings. The elaborate choreog- 
raphy of Saturn’s complex ring system is produced and orchestrated by the 
combined gravitational tugs of Saturn, its moons that lie out beyond the 
rings, and even the neighboring ring particles on each other, as well as by soft 
collisions between nearby bergs and particles. 

★ The ring particles range in size from smaller than sugar grains to as 
large as houses. They are believed to resemble irregular snowballs — not as 



Figure 13-8. An artist's rendition portrays striking features and the startling 
complexity of the vast Saturnian ring system , as though woven into an intricate 
space tapestry. 

hard as ice cubes or as fluffy as cotton candy. The smaller particles far 
outnumber the larger ones, but the entire mass of the rings is not more than 
that of a moon whose diameter is only 320 km (200 mi). 

★ The main rings cover an area of just over 40 billion square km (15 
billion square mi), or roughly 80 times the total surface area of the Earth! To 
journey radially away from Saturn from the innermost to the outermost edge 
of the main rings, a space traveler would need to cover a distance equal to 13 
times that across the United States. Tip to tip, they span roughly 70 percent 
of the distance from the Earth to the Moon. However, the thickness of the 
ring sheet would rarely exceed 10 m (33 ft), though corrugations in the sheet 
would rise and fall by as much as 1.6 km (1 mi). The Saturn ring sheet is so 
thin that, were a scale model to be made from material 
as thick as a quarter, its diameter would have to be 1 6 
km (10 mi). 

★ Some scientists believe that the rings consist of 
primordial debris that has been unable to coalesce 
into proper moons because it lies within the Roche 

TIME: 0 Days. 3:45:00 
1989/08/25 7:45 GMT (SCT) 




103437 KM 

limit — the boundary separating the “safe” zone for satellites from the region 
where, conditions permitting, the planet's gravity can literally pull satellites 
apart. Others suggest that the rings are former moons that have been 
captured by Saturn, broken apart, further chipped away by collisions among 
the pieces, and spread out in a thin disk around the planet. 

Still other scientists offer the most appealing explanation of all for the 
origin and maintenance of the rings: during the later stages of Saturn's 
formation, one or more moons formed outside the planet’s Roche limit. But, 
as Saturn continued to pull in matter, the Roche limit moved out beyond 
these moons, which were small enough and hard enough to avoid destruction 
by Saturn's strong gravity. Instead, they were eventually shattered by 
impacts from incoming meteoroids. The latter hypothesis suggests that, 
even now, other “ringmoons,” smaller than their ancestors but a thousand 
times larger than the largest ring particles, are patiently orbiting as they 
have been for millions of years, awaiting that moment when they too will be 
transformed into magnificent rings. 

Saturn’s Moons 

★ Saturn may be the most prolific planet in the solar system, producing 
at least seventeen offspring, with evidence for several more. One of these, 
Phoebe, may actually have been a wandering stray from the asteroid belt or 
elsewhere that Saturn adopted into its extended family. 

★ The five largest moons — Tethys, Dione, Rhea, Titan, and Iapetus — 
range in diameter from 1060 to 5150 km (650 to 3200 mi), and were all 
discovered telescopically in the seventeenth century. The eighteenth cen- 
tury can lay claim to Mimas (392 km or 244 mi) and Enceladus (500 km or 
310 mi). By the end of the nineteenth century, Hyperion (410 by 260 by 220 
km or 255 by 160 by 140 mi) and Phoebe (220 km or 140 mi), the only two 
satellites of Saturn which are not in locked rotation (i.e., rotating about their 
spin axis such that they keep their same face toward Saturn), joined the 
ranks. Saturn's remaining known satellites (eight total) range in diameter 
from 25 to 190 km (15 to 120 mi); all are non-spherical, and all are smaller 
than the others mentioned above. All were discovered in the twentieth 
century, including three discovered by Voyager 1 during its 1980 encounter 
with Saturn. 

★ Titan, the first of Saturn's moons to be discovered (by Huygens in the 
early 1600s), is classified as a moon even though it is larger than Mercury. 
Its surface is hidden from view (except at infrared and radio wavelengths) by 




Figure 13-9 . Titan is the only satellite in the solar system that has an atmosphere 
more substantial than Earth ’s. Titan's atmosphere is composed mostly of nitrogen 
(like the Earth's), but hydrocarbons slowly rain upon seas of ethane that are 
believed to cover much of the frigid surface that lies below the thick cloud and haze 
layers. The mysteries below Titan's clouds will hopefully be revealed by the 
Cassini probe to Saturn in early 2003. 

TIME: 0 Days. 3:50:00 
1989/08/25 7:50 GMT (SCT) 

a dense atmosphere. This latter characteristic makes it unique among all 
moons in the solar system and the subject of much scientific curiosity. 

Titan’s primarily nitrogen atmosphere is more like 
Earth’s than the atmospheres of Venus or Mars, 
which are composed primarily of carbon dioxide, or 
like Jupiter, Saturn, Uranus, and Neptune, which 
have atmospheres composed primarily of hydrogen 
and helium. Methane and argon are probably the 
other main constituents in Titan’s atmosphere. 


98399 KM 


' -WM V& 

Titan might appear an interesting place to spend a holiday, since it may 
have an ocean covering its surface to a depth of 1 km, with islands of solid 
water ice rising from the ocean floor. Of course, what the travel brochure 
would probably fail to mention is that this “island paradise” would be atop 
an ocean of organic sludge of ethane, methane, and nitrogen, and that its 
frigid temperature of -179°C (-290°F) would be most uncomfortable. 

The planned Cassini mission to Saturn in the late 1990s perhaps will 
end all false claims to Titan’s desirability as a vacation destination when it 
drops a probe through Titan’s thick clouds and haze to reveal, for the first 
time, the details of its true ambiance. 

★ Tethys is one of two Saturnian satellites to be a “parent.” (Dione is the 
other.) Tethys plays guardian to the Lagrarigian satellites, Calypso and 
Telesto. Lagrangian satellites are small objects that, by maintaining 
approximately 60 degrees of arc ahead of or behind a larger parent object, can 
share the same orbit and orbital speed as the parent. Tethys has an 
unusually large impact crater ( named Odysseus) on its leading hemisphere 
centered at about 30° north latitude. 

★ Dione is distinguished by bright wispy markings that resemble thin 
veils covering its soft features. The veils probably were created by frost-like 
material formed by the explosive release of volatiles from its interior. It is 
estimated that all of Saturn’s satellites from Dione inward were struck at 
least once by a body with sufficient kinetic energy to shatter the satellite. 
Therefore, it is likely that Dione and the inner moons have either been 
reassembled in orbit or are mere fragments of their former selves. 

★ Most of the known moons in the solar system have certain eccentrici- 
ties and unique features, but Iapetus exhibits the most split personality. Its 
light-to-dark contrast is the most extreme yet seen in the solar system: its 
dark face is as dark as an asphalt parking lot, while its light face is as white 
as a fresh blanket of snow. The dark surface has sharply defined edges and 
contains no visible impact craters, suggesting that the dark material may 
have formed after methane flowed from the interior. (Methane turns very 
black after having its carbon-hydrogen bonds broken by sunlight or radia- 
tion bombardment.) 

★ Unlike high-contrast Iapetus, Phoebe is one of the darkest objects in 
the solar system. It is conjectured that Phoebe is a wayward asteroid that 
was adopted (or kidnapped) by Saturn. If so, Phoebe is the first asteroid 


observed in the outer solar system from a relatively nearby spacecraft. 
Phoebe's asteroid nature is supported by the fact it is the only Saturnian 
satellite in a retrograde orbit (i.e., it orbits opposite the rotation direction of 
the main planet). It is also inclined by about 30° with respect to Saturn’s 

★ Hyperion is probably a mere shadow of its former self. This dark, 
irregularly shaped satellite is thought to be a remnant of a much larger object 
which was shattered by impact. It orbits about Saturn in a random-like 
motion described as “chaotic tumbling.” 

★ Although Mimas and Enceladus are similar in diameter and in 
proximity to Saturn, they are in marked contrast with each other. Mimas 
has a huge impact crater (Herschel) nearly one-third of its diameter. The 
crater is 10 km (6 mi) deep with a central peak rising 6 km (4 mi) from its 
floor. Mimas came close to being shattered by the impact that created this 
huge pockmark. In contrast, portions of Enceladus’s surface are nearly 
devoid of impact craters. Enceladus shows evidence of a complex geological 
history unsuspected for such a small object. Enceladus may experience 
active water volcanism. As its orbit is coincident with the densest part of 
Saturn’s tenuous E-ring, Enceladus may be the source of the E-ring material. 

Saturn’s Magnetosphere 

★ Prior to the Voyager encounters, Saturn was shown by Pioneer 11 to 
have a magnetic field. The field is basically dipolar in nature, having well- 
defined north and south magnetic poles, and is aligned with Saturn’s axis of 
rotation to within 1 degree. 

★ Sunward of Saturn, the magnetopause varies in location from less 
than 14 to more than 30 Saturn radii*, depending upon the intensity of the 
solar wind. 

★ Inside 7 Saturn 

TIME: 0 Days, 3:55:00 

radii there is a torus of hydrogen and oxygen ions, 
possibly originating from the sputtering of water ice 
from the surfaces of Dione and Tethys. There is also 
a doughnut-shaped region of neutral hydrogen atoms 
extending from 8 to 25 Saturn radii, which probably 
originated from the atmospheres of Titan or Saturn. 

*Saturn’s equatorial radius is 60,268 km (37,450 mi), at a pressure 
of 1 bar. 




93415 KM 




Though a variety of names were suggested for this planet shortly after 
its discovery in 1781, the name Uranus prevailed. In Roman mythology, 
Uranus was the father of Saturn, god of the sky, and husband of Earth. Now, 
Uranus takes on the added role of being yet another member of the planetary 

By then a seasoned traveller, Voyager 2 began its tour of the Uranian 
system on November 4, 1 985. For the following 3-1/2 months, and for the first 
time in the history of our species, Uranus provided us with some tantalizing 
clues as to the “sleight of hand” behind his magic. 

Uranus’ equator is tilted 98 degrees with respect to the plane of its 
orbital motion about the Sun. The most widely accepted reason for this 
strange inclination is that the planet was struck off-center by a body roughly 
one to two times the size of the Earth. As this body broke up and became part 
of Uranus, it’s suspected that it imparted enough rotational energy to the 
planet — which at the time was probably spinning more “right-side up” — to 
reorient its spin axis. This hypothesis remains conjecture, however, since 

Figure 13-10. Two small satellites (Prometheus and Pandora) “shepherd” Saturn’s 
F-ring between them. These satellites confine the ring particles into a narrowband , 
and may have a role in the “braiding” in the F-ring observed by Voyager 1. 


other evidence for such an impacting body has not been seen anywhere in the 
Uranian systems 4 , * . 

One of the more surprising outcomes of this encounter was the devel- 
opment of a new interior model for Uranus based upon a comparison of its 
longer- than-expected rotation period with its previously determined gravity 
field. Prior to this time, it was suggested that Uranus’ interior consisted of 
three distinct layers: (1 ) a rocky core containing mostly magnesium silicates 
and iron, covered by (2) an “oceanic” mixture of water and other constituents, 
which in turn was enclosed by (3) a molecular layer consisting mostly of 
hydrogen. The new post-Voyager models discard previous models and 
propose instead a structure having a large core composed of a mixture of rock, 
ice, and gas, covered by a thick atmosphere of hydrogen, helium, and heavier 
gases. The core either has many layers with different compositions or else 
is characterized by a continuous density and composition gradation within 
the core. A very small molten “rocky” core may or may not exist at the 
planet’s center. 

In contrast to Jupiter and Saturn, Uranus has at best a weak internal 
heat source — less than 1 3 percent of its radiated heat comes from its interior. 
It is possible that all of its observed heat is provided by the Sun. 

Uranus' Atmosphere 

★ Due to Uranus’ unusual inclination, the polar regions receive more 
sunlight during a Uranus year (84 Earth years), and scientists anticipated 
that its poles would be about 4°C (7°F) warmer than its equator. The 
Uranian winds were also expected to be different than those found at less- 
inclined planets such as Saturn. Instead, Voyager 2 discovered that the 
equatorial temperatures at Uranus are remarkably similar to temperatures 
at the poles (-209°C or -344°F), implying that some redistribution of heat 
toward the equatorial region must occur within the atmosphere. And, the 
wind patterns are much like Saturn’s, flowing parallel to the equator in the 
same direction as the planet’s rotation. Thus, like at Saturn, circulation 
patterns at Uranus are apparently determined chiefly by the effects of the 
planet’s rotation, rather than by the distribution of sunlight on the planet. 

TIME: 0 Days. 4:00:00 
1989/08/25 8:00 GMT (SCT) 

★ Ninety-eight percent of the upper atmosphere of 
Uranus is composed of hydrogen and helium; the 
remaining 2 percent is methane. However, scientists 
speculate that the bulk of the lower atmosphere is 
composed of water (perhaps as much as 50 percent), 
methane, and ammonia. (Water and ammonia were 



88493 KM 


Figure 13-11. Uranus is surrounded by its five largest satellites in this 
montage of images from the January 1986 encounter. The planet is also 
encircled by eleven rings , but they are too dark to be seen in this montage. 

not detected during the encounter because they freeze at much deeper levels 
in the atmosphere than could be detected by Voyager 2.) Methane, which is 
responsible for Uranus’ blue-green color because it selectively absorbs red 
sunlight, condenses to form clouds of ice crystals in the cooler, higher regions 



of Uranus’ atmosphere. The clouds, which generally are useful in tracking 
the Uranian wind patterns, are optically thin and nearly featureless. 
Compared to the tumultuous clouds detected at Jupiter, the Uranian clouds 
are relatively calm, and no lightning was observed by Voyager 2. 

Uranus' Rings 

★ Two additional rings (and an extensive set of dust bands) were 
discovered during the Uranus encounter, which brings the total number of 
known rings to eleven. (The first nine were discovered during a star 
occultation event observed from Earth relatively recently, in 1977.) The 
rings all lie within one p lanetary radius* of Uranus’ cloud tops. They are, in 
order of decreasing orbital radius from the center of the planet, epsilon, 
1986U1R (the first of the newly discovered rings), delta, gamma, eta, beta, 
alpha, 4,5,6, and 1 986U2R (the second of the newly discovered rings). Bands 
of dust- sized ring material extend inward from 1986U1R, perhaps all the 
way to the planet. Astronomers in India probably saw 1986U2R during the 
1977 occultation event, but since no other observatory did, the claim could 
not be confirmed. We could say, then, that Voyager 2’s “discovery” of this ring 
was really a confirmation. 

★ Except for 1986U2R and the dust bands, Uranus’ rings are all narrow, 
much like some of the narrow rings observed at Saturn. They range in width 
from 1 to 93 km (0.6 to 58 mi), and are only a few kilometers thick. The ring 
structure appears to vary considerably with longitude. For example, the 
outermost Uranian ring, epsilon, is nearly five times as wide at its most 
distant point from the planet as it is at its closest point. 

★ The Uranian rings are colorless and extremely dark. The dark material 
may be either irradiated methane ice or organic-rich minerals mixed with 
water-impregnated, silicon-based compounds (something akin to carbon- 
type asteroidal material). If compressed together, the Uranian ring material 
would form a moon about 30 km (19 mi) across. 

TIME: 0 Days, 4:05:00 

★ Based upon observations of the narrow rings of 
Saturn, it was expected that the Uranian ring system 
would consist of a large quantity of dust; it was a great 
surprise to learn that dust comprises less than 1 
percent of the Uranian system. (The rings primarily 

*The equatorial radius of Uranus is 25,560 km (15,880 mi), at a 
pressure of 1 bar. 




83644 KM 



Figure 13-12. An extensive distribution of dust is seen within the Uranian ring 
system in this Voyager-2 image taken in forward-scattered sunlight. The short 
streaks are stars smeared out by Voyager 2’s motion while the camera shutter was 

consist of particles with diameters of several centimeters.) The mechanism 
which sweeps the rings clear of such planetary dandruff appears to be 
Uranus' extreme upper atmosphere, which is considerably more dense than 
expected. The dislodged dust particles appear to take up temporary resi- 
dence in bands at or beyond 39,500 km (24,500 mi) from the center of the 
planet. At closer distances, the particles eventually fall into Uranus and 
blaze out of existence as meteorites. 

★ One of the mechanisms thought to keep ring particles from drifting off 

into space or towards the planet is what has come to be known as shepherd- 
ing. Shepherding satellites are tiny moons, first postulated to explain the 
Uranian rings, but first observed during Voyager 1 ’s encounter with Saturn. 
They orbit close to the rings, gravitationally nudging wayward particles back 
inside their proper borders. Two such shepherds were imaged flanking 
Uranus' epsilon ring. Additional shepherding satellites were not imaged 


near any of the inner Uranian rings, possibly because the moons, if they 
exist, are too dark and/or too tiny to have been sensed by Voyager. A possible 
shepherd has been inferred to exist 9 km inside the eta ring based upon RSS 
and PPS detections of density waves in the delta ring. 

★ Incomplete rings or “ring arcs,” similar to those discovered within the 
Encke division in Saturn's rings, may have been seen by the PPS at Uranus 
during the encounter. There is evidence that ring arcs exist at Neptune as 
well. Scientists are curious about the source of these partial rings and the 
mechanisms that maintain their structure. They may be transient clumps of 
debris arising from collisions between moons orbiting near the planet in belt- 
like groups, or, alternately, clumps of debris maintained in place by one or 
more (as yet undiscovered) moons orbiting in inclined orbits. 

Uranus' Moons 

★ Ten additional Uranian moons were discovered by Voyager 2, bringing 

the total number of known moons there to 1 5. In order of decreasing distance 
from the planet, the 15 moons are Oberon, Titania, Umbriel, Ariel, Miranda, 
Puck, Belinda, Cressida, Portia, Rosalind, Desdemona, Juliet, Bianca, 
Ophelia, and Cordelia. 

The first five moons listed were discovered prior to the Voyager-2 
encounter, the most recently discovered being Miranda, found by Gerard B. 
Kuiper in 1948. Oberon and Titania were discovered by William Herschel, 
the discoverer of Uranus, in 1787. Herschel may also have spotted Umbriel 
on April 17, 1801. The fi rst definitive sightings of Umbriel and Ariel were 
reported in 1851 by the English amateur astronomer William Lassell, who 
had discovered Neptune’s moon Triton four years earlier. There is some 
evidence that Ariel was first sighted by Otto Struve on October 8, 1847. 

In 1852, John Herschel, William’s son, borrowed names from English 
literature (works of Shakespeare and Pope) to name the first five known 
moons, breaking the tradition of using names from Greek or Roman mythol- 
ogy. The naming of the moons found by Voyager 2 followed the younger 
Herschel’s precedent. 

TIME: 0 Days, 4:1 0:00 
1989/08/25 8:10 GMT (SCT) 

★ The two largest moons, Oberon and Titania, 

are less than half the diameter of Earth’s moon. Both 
moons have average densities between 1.6 and 1.7 
gm/cm 3 , surprisingly high in comparison to Saturn’s 
icy moons. Scientists anticipated that as one pro- 




78880 KM 



Figure 13-13. Ten newly discovered moons at Uranus in their positions between the 
rings and Miranda at the time three hours before Voyager 2’s closest approach. 
(Computer graphics representation; names chosen by the International Astronomical 
Union [IAU].) 

gressed farther from the warmth of the Sun, bodies would be icier, and thus 
have lower densities. 

★ Titania, the reddest of Uranus’ moons, may have endured global 
tectonics as evidenced by complex valleys and fault lines etched into its 
surface; smooth sections indicate that volcanic resurfacing has taken place. 
(All four of Uranus' largest moons have been completely resurfaced.) Ti- 
tania's surface fracturing may have been caused by the expansion of frozen 
subsurface water. Another possible explanation for its wrinkled appearance 
is that at one time in its history it was blasted apart by a large impacting body 
and subsequently reassembled in its present orbit. Oberon, however, shows 
few signs of having experienced tectonic activity since visible fault lines are 
nearly absent on its heavily cratered surface. 

★ Umbriel and Ariel are roughly three-fourths the size of Oberon and 
Titania. Umbriel is the darkest of Uranus' large moons (19 percent reflec- 
tance) with huge craters peppering its surface. Unlike its sisters, Umbriel 


has a paucity of what are known as bright ray craters, which are formed on 
an older, darker surface when bright submerged ice is excavated and 
sprayed by meteroid impacts, a process sometimes referred to as “impact 
gardening.” One hypothesis for Umbriel’s dark surface is that it is the 
original surface, unchanged over the eons except by infrequent large mete- 
oroid impacts. Or, the deep gray surface on Umbriel may be due to a fairly 
recent coating of material from an unknown source — presumably a nearby 
source, since Titania and Ariel, the moons on either side of Umbriel, are not 
coated. A plausible candidate for such a body would be a carbon-rich 

In contrast, the surface of Ariel, the brightest of the Uranian moons, is 
relatively free of pockmarks due to a self-repair mechanism commonly 
known as volcanism, which periodically erases the damage done by foreign 
projectiles. But this process has acted incompletely there, leaving several 
extremely deep (tens of 1cm) cuts on Ariel’s surface. 

★ The smallest of Uranus’ large moons, Miranda, was aptly described 
during the minutes immediately following its high-resolution debut in 1986 
as “the most bizarre body in the solar system.” Considering its size (only 
about one-sixth the diameter of Earth’s Moon), Miranda has a remarkable 
variety of terrain: rolling, heavily cratered plains (the oldest known terrain 
in the Uranian system) adjoined by three huge, 200 to 300 km (120 to 1 80 mi) 
oval-to-trapezoidal regions known as coronae, which are characterized by 
networks of concentric canyons. The coronae, volcanic complexes named 
Arden, Elsinore, and Inverness, are the subject of great interest due to their 
complex geometry: all are much less cratered than the plains and contain an 
oddly oriented series of ridges, grooves, and cliffs of differing reflectances 
and dimensions. 

What could possibly explain Miranda’s bizarre surface features? One 
interesting hypothesis accepted by a minority of scientists is that on several 
occasions the moon was impacted, shattered, and haphazardly slapped back 
together gravitationally. The more popular hypothesis is that the surface of 
Miranda was breached by huge slabs of ice that rose upwards during its 

TIME: 0 Days. 4:15:00 
1989/08/25 8:15 GMT (SCT) 

initial period of differentiation (settling into layers). 
But the differentiation process was halted before 
completion as the primordial heat ran out, and Mi- 
randa was left like an abandoned sculpture. 

★ Nine of the new moons range in size from 26 

to 108 km (16 to 67 mi) and, being closer to the planet, 



74217 KM 

Figure 13-14. This image of Uranus’ moon Miranda — the highest-resolution image 
ever returned by either Voyager spacecraft— shows the bizarre variety in the 
geology found there by Voyager 2. The smallest discernible features are less than 
1 km (0.6 mi) across. 

have faster periods of revolution (8 to 15 hr) than their more distant 
relatives. Puck, the tenth and largest new moon, and the first to be 
discovered by Voyager, is 154 km (96 mi) in diameter and makes a trip 
around Uranus every 18 hours. 

★ Two of the moons discovered by Voyager, Cordelia and Ophelia, are 
shepherding satellites that gravitationally constrain the outermost ring, 
epsilon. The other eight new moons lie between the orbits of Ophelia and 



★ Puck and Cordelia were the only two new moons whose disks were 
resolved in Voyager imaging; each reflects only 7 percent of the incident 
sunlight. Puck is shaped somewhat like a potato, with a huge impact crater 
marring roughly one-fourth of its surface. Compared to Puck and Cordelia, 
the five first-known Uranian moons are not nearly as dark — they reflect 
from 19 to 40 percent of the incident sunlight. They are, however, darker 
than their Saturnian counterparts. It is speculated that their darkness 
relative to Saturn’s moons may be due to a surface coating of carbon-rich 
organic substances. This hypothesis relies on the fact that all of Uranus’ 
moons spend a large fraction of their orbits within the magnetosphere, 
bombarded by energetic protons which can darken their surfaces through 
irradiation of the methane ice thought to comprise a large portion of each 
moon. Another difference between the Uranian and Saturnian moons is that 
Uranus’ moons are brightest in areas where there are geologic features, 
suggesting that perhaps sub-surface ices oozed or leaked through newly 
formed cracks and craters. 

★ For years to come, the moons of Uranus will continue to bedazzle 
planetary geologists. Of particular interest is how bodies the size of Miranda 
and Ariel could undergo tectonic and volcanic processes — once thought to be 
limited to large bodies which possess the capability of generating sizeable 
internal heat reserves. Two of the hypotheses currently being bantered 
about are that the requisite heat sources arise from (1 ) the tidal heating that 
occurs when a close satellite orbits its planet in an elliptical path (tidal 
heating is thought to be the source of geological activity on Io and Europa at 
Jupiter, and Enceladus at Saturn), or (2) heating from radioactive materials 
trapped in crystals of water ice. 

Uranus' Magnetosphere 

★ Prior to the Voyager-2 encounter, it was unknown whether Uranus 
had a magnetic field, due to the absence of nonthermal radio emissions in 
previous observations. To the delight of the scientists, five days before 
closest approach Voyager 2 discovered that a magnetic field indeed did exist 

at Uranus. Of great surprise was the discovery that 
the magnetic axis can be represented by a dipole tilted 
58.6 degrees with respect to Uranus’ rotation axis, by 
far the greatest offset seen at any of the planets with 
magnetic fields. (Earth’s magnetic axis has the sec- 
ond largest inclination, 11 .4 degrees; Saturn’s has the 


TO TRITON: 69674 KM 

TIME: 0 Days, 4:20:00 


smallest, 0 degrees. )The inclination of the Uranian magnetic axis causes the 
field to wobble as the planet rotates. 

Another misalignment discovered with regard to this field is that the 
magnetic center of the planet is displaced from the planet's center by 0.3 
Uranian radii. (At Earth, this displacement is only 0.08 Earth radii; at 
Saturn, it is only 0.02 Saturnian radii.) One possible explanation for both of 
these anomalies is that the magnetic field is undergoing a reversal; however, 
this explanation, although supported by the fact the Earth has undergone 
several field reversals, is by no means conclusive. One hypothesis suggests 
that conditions in the interior of Uranus permit more rapid reorientation of 
the magnetic field, which may be experiencing long-term, semi-periodic 

★ The magnetosphere is formed into a giant wind-sock shape around 
Uranus by the incoming solar wind, with a huge tail at least 42 times longer 
than Uranus' radius extending from the planet's dark side. The magnetotail 
is very similar to Earth's. In fact, other than its high tilt and large offset, 

Figure 13-15 . A computer-graphics reconstruction of the surprising orientation of 
Uranus 7 magnetic field. The planet's spin axis is shown pointing towards the Sun 
to the left ; the north pole of the magnetic dipole points to the upper left. 


Uranus' entire magnetosphere appears to be more Earth-like than Jupiter's 
or Saturn’s. Another feature Uranus' magnetosphere shares with Earth’s is 
the presence of radiation belts (Van Allen belts), which are concentrated 
regions of high-energy charged particles. 

Cruise Science Results 

Like the Olympics, the Voyagers’ planetary encounters have provided 
the public with an exhilarating experience that comes but once every several 
years. Like the Olympic torch, the Voyagers have fired our imaginations, 
and like the Olympic athlete, the Voyagers have no respite between their 
brief moments of sublimity. 

The cruise phase of the Voyager missions is not as restful as the word 
“cruise” may imply. Both Voyagers have made a multitude of important sci- 
entific observations during these periods. Few spacecraft have ventured 
beyond the inner solar system (only Pioneers 10 and 11 besides the Voyag- 
ers), so the Voyagers have provided information regarding the far “outback” 
which otherwise would be lacking. 

During cruise the Voyagers primarily perform research in three areas: 
(1) ultraviolet observations of both stars and the interstellar medium using 
the Ultraviolet Spectrometer (UVS), (2) asteroid-belt dust and planetary 
atmosphere observations with the Imaging Science Subsystem (ISS), and (3) 
fields and particles research, employing a variety of Voyager instruments. 

Ultraviolet Observations 

Stellar observations in the extreme ultraviolet (EUV) wavelengths 
from 500 to 912 angstroms and the far ultraviolet (FUV) wavelengths from 
912 to 1200 angstroms are routinely performed during cruise with Voyager’s 
UVS. The only other methods currently available for ultraviolet research 
employ aging Earth-orbiting spacecraft (such as the International Ultravio- 
let Explorer, or IUE), sounding rockets, and balloon-borne instruments, but 
these instruments do not cover the EUV part of the spectrum (until the Ex- 
treme Ultraviolet Explorer satellite, or EUVE, is launched in 1991). Using 
data from its deep-space perspective, Voyager has caused us to ask more 
questions than we have been able to answer. The 
time: o Days. 425:00 pieces of the UV puzzle don’t quite fit like we expected. 

1989/08/25 8:25 GMT (SCT) 

★ White dwarfs represent the senior citizens 

of relatively small stars (up to about 5 times the mass 
of the Sun). Having spent the last several billion 
years consuming its nuclear-energy resources, the 




65276 KM 

white dwarf has gradually contracted to a body about the size of the Earth 
while retaining a significant fraction of its original mass. The Voyagers have 
provided temperature and atmospheric readings for many members of this 
stellar community. The temperatures of the newest residents exceed by as 
much as 7 0,000°C (126,000°F) the thermal energy that can be accurately 
measured from Earth. Also, severe constraints on the radius and tempera- 
ture of Sirius B, the white dwarf companion to Sirius (the brightest star 
visible to the naked eye from Earth's northern hemisphere), have been 
provided by Voyager during cruise, in conjunction with observations by an 
X-ray satellite named XOSAT. 

★ Another class of stars observed by Voyager during cruise is the Beta 
Cepheid variables. The distinctive feature of these hot stars is that from four 
to six times a day their brightnesses vary. Only Voyager has been able to 
accurately measure their energy distributions, since the majority of the 
radiation they emit is in the EUV band. 

★ Cataclysmic variables, as the name implies, also exhibit varying 
degrees of brightness. However, unlike the Beta Cepheid variables, the 
Cataclysmics are binary star systems, flaring more brilliantly and far less 
frequently than the Beta Cepheids. Voyager observations have disproved 
hypotheses that proposed strong EUV and FUV emissions during these 
outbursts, and support hypotheses that suggest that these outbursts origi- 
nate from a disk of material orbiting the white dwarf member of the system. 

★ Stellar observations with Voyager’s UVS additionally have provided 
information regarding the variability of (1) many O-type (hottest) and B- 
type (second hottest) stars, (2) the physical conditions prevailing in and the 
mechanisms behind the gaseous envelopes surrounding B-type stars, (3) 
supernova remnants including the Cygnus Loop, which represents the 
remnant of a star that exploded 50,000 years ago in the constellation Cygnus, 
and (4) the influence of the Sun on both the Earth and planetary atmospheres 
through Voyager’s monitoring of solar EUV/FUV variations. 

★ The UVS on Voyager has provided the best measurements on the FUV 
component of the interstellar radiation field which originated in the early 
universe. The density, temperature, and electrical state of the local inter- 
stellar material and the scattering properties of interstellar dust have been 
the subject of UVS cruise observations as well. 


★ Also, the UVS has observed objects beyond the Milky Way during 
cruise, including quasars, the brightest and most distant known objects in 
the universe. To explain the immense amount of radiation coming from the 
quasars, it has been suggested that a massive black hole is situated at the 
center of each quasar, which is in turn located within a galactic core. 

Imaging Science 

★ In late 1983, the Earth-orbiting Infrared Astronomical Satellite (IRAS) 
discovered bands of dust encircling the solar system immediately above and 
below the ecliptic plane. Further analysis revealed that the bands are 
actually doughnut-shaped (vs. disk-shaped) due to influences on the par- 
ticles' orbits arising from Jupiter’s gravitational pull. It is speculated that 
the bands consist of roughly 5 trillion tons of pulverized asteroidal material 
resulting from collisions within the asteroid belt. Observations of forward 
scattering of the dust particles by Voyager l’s wide-angle camera have 
provided upper limits on the thickness and total area of the dust bands; these 
data support estimates derived from the IRAS data. 

★ Voyager imaging of Jupiter, Saturn, Uranus, and Neptune at different 
phase angles has provided invaluable information regarding the outer 
planets’ internal heat sources. Unlike the terrestrial planets, the outer 
planets do not radiate roughly the same amount of energy as they absorb 
from incoming sunlight. More than 88 percent of the heat emanating from 
Uranus, 56 percent of the heat from Saturn, 60 percent of the heat from 
Jupiter, and (possibly) 48 percent of the heat emanating from Neptune 
comes from the Sun. These observations have defined the variation of each 
planet’s brightness as the angle of the illuminating sunlight varies. Infor- 
mation about their internal heat has important implications for the origin, 
evolution, internal structure, and meteorology of the outer planets. 

Fields and Particles Research 

★ During cruise, the Voyagers have participated with Pioneers 1 0 and 1 1 
in gathering information regarding the heliosphere in the distant reaches of 

TIME: 0 Days, 4:30:00 
1989/08/25 8:30 GMT (SCT) 

the solar system (all four spacecraft are currently 
beyond 28 times the Sun-to-Earth distance). The 
heliosphere is the region around our Sun which is 
dominated by the solar wind. The solar wind, consist- 
ing of super-heated (roughly 100, 000°C, or 180,000°F) 
ionized gases, travels outward from the solar corona 
at approximately 400 km/sec (864,000 mph). Thus 



61055 KM 

far, Voyager observations have shown that the solar wind speed is constant 
to at least Uranus’ orbit in one direction; Pioneer 10 has shown that it is 
constant to at least that far in the opposite direction. The solar magnetic field 
eventually forms into a spiral due to the rotational effects of the Sun, much 
like the pattern that emanates from a rotating sprinkler head. 

★ As the solar wind continues its journey beyond the Sun, its structure 
and dynamics evolve. At a distance of 50 to 150 times the Sun-Earth 
distance, the solar wind finally slows to subsonic speeds and interacts more 
intimately with the Sun’s magnetic field, which interacts with the "stellar 
winds” from nearby stars. When the pressures of the solar wind and 
interstellar plasmas are equal, a relatively thin boundary region, known as 
the "heliopause”, forms. Since 1979, both Voyagers have detected certain 
low-frequency interplanetary radio emissions which indicate that they are 
approaching some type of boundary that may be as close as the outer reaches 
of Pluto’s orbit. This boundary may represent the "terminal shock,” the 
region in which shock waves are created from the initial interaction of the 
placid interstellar medium with the supersonic solar wind. Data obtained 
from the spacecraft when they finally enter this region will expand our 
limited knowledge of the characteristics of the shock geometry as well as 
place an accurate limit on the size of the heliosphere. 

★ The data obtained by the spacecraft during cruise have greatly en- 
hanced our understanding of not only the dimensions of the heliosphere and 
the structure of the interplanetary medium, but also the character and 
transport of energetic particles originating from outside the solar system. 
Cosmic rays traveling at significant fractions of the speed of light enter the 
heliosphere and interact with the Sun’s magnetic field. The Voyager 
energetic particle experiments have measured the variation in the galactic 
cosmic ray intensity as the spacecraft has moved away from the Sun. These 
results can be used to infer, in principle, the distance to the heliopause. 

★ The research the Voyagers have performed during cruise will continue 
during the Voyager Interstellar Mission (VIM), as discussed in Chapter 12. 
The body of knowledge created by Voyager observations will not only provide 
a better understanding of the heliosphere, but also the interstellar/galactic 
environment, within which the heliosphere, and humanity, reside. 


TIME: 0 Days, 4:35:00 




57047 KM 


Good-night, good-night! Parting is such sweet sorrow, that 

I shall say good-night till it be morrow . 

William Shakespeare 



The Voyager mission and its many exciting discoveries have touched 
the minds, imaginations, and hearts of many people. The greatest impact 
has probably been felt by those Voyager personnel who have dedicated a 
large fraction of their professional careers to making Voyager a successful 
mission of exploration. These are the people who have worried about 
Voyager each day, for many years, often outside of the normal eight-hour 
work shift, sometimes awaking during the wee hours to jot down an item to 
check out the next day. 

Voyager has also touched the thoughts and lives of artists, writers, 
educators, and many others who have followed the mission's progress 
through articles in popular magazines and newspapers. Many of these 
people are widely known, and they too feel strongly about the Voyager 
achievements and the future hopes spawned by these two intrepid robots. 

Therefore, several people were invited to reflect upon the meaning of 
the Voyager mission, but they were limited to several dozen words or less. 
Furthermore, as it was not practical to contact the more than 3000 people 
who have worked on the Voyager mission at some time over the past 1 7 years, 
it was necessary to choose several of their leaders to act as spokespeople. 
Outside of the Voyager Project, several well-known people were also invited 
to express their reflections in this chapter of the Guide. Without further ado, 
arranged alphabetically by contributor, the following quotes are offered: 

Voyager Personnel 

This fall, almost exactly twelve years after launch, Voyager will 
make its last planetary encounter. For all of the experimenters the 
mission has represented a unique opportunity for exploratory research 
in his or her chosen field of planetary science. It is also a demonstration 
of a unique collaboration between the scientific community and JPL 
and the ability of this combination to exploit fully a one-time scientific 
opportunity to explore the outer planets. 

Herb Bridge 
Principal Investigator 
Plasma Investigation 


Voyager and its successes have been a direct result of capable, 
dedicated people operating in a unique team environment. I credit 
Bud Schurmeier with creating the Voyager team spirit through his 
careful, deliberate selection of the original team members and his 
attention to developing the team spirit with his management of the 
Project. It was a pleasure to have worked with Bud Schurmeier on 
Voyager. It has been a credit to subsequent Project Managers that the 
team spirit and the excellence of the effort have been sustained. 

Ray Heacock 
Project Manager 

Even in hindsight, I would change not one whit of the Voyager 
experience. Dreams and sweat carried it off. But most of all, its legacy 
makes us all Earth travelers among the stars. 

Charley Kohlhase 
Mission Planning Manager 

How was I to know as a kid growing up on a Greek island and 
watching the planets in the clear, dark sky that some day I would be 
part of the select group of people from Earth that put together the 
mission to take a close-up look at four of the five outer planets ? This 
has to be the stuff that dreams are made of! 

Tom Krimigis 
Principal Investigator 
Low-Energy Charged Particle 



53301 KM 


I have no desire to do much else except to ride this thing all the way 
out into interstellar space. 

Dick Laeser 
Mission Director 
Project Manager 

To me, the Voyager Project epitomized the salient and best 
features of America as a country and democratic society. The Project 
was conceived and implemented during the most vigorous and 
healthiest days of the national space effort. Some of the most creative 
and imaginative personnel were given the well-defined goal to 
advantageously utilize a unique celestial circumstance of planetary 
alignment which permitted realizing a goal previously thought about 
only in science fiction, namely “A Grand Tour of the Four Giant Outer 
Planets: Jupiter, Saturn, Uranus, and Neptune.” 

Truly, the Voyager can be considered to have been the scientific 
space mission of a lifetime! All of us associated with the Project share 
in its glory , as does the United States. I take this opportunity also to 
recognize the many dedicated workers, whose tender loving care 
created such a unique artifact with which to extend the senses of 
mankind in our universal search for truth and understanding. 

Norman Ness 
Principal Investigator 
Magnetic Fields Investigation 

From the very beginning, Voyager had a very special aspect to it: 
a uniqueness, a challenge, a promise, an appeal that has made it a 
never-to-be-forgotten experience for those of us who have had the good 
fortune to be closely involved. It was born out of adversity but, as time 
has now proven, it was very solidly based and exceedingly well- 


Bob Parks 
Project Manager 

In 1971, when we started work on the Grand Tour missions, we 
had the dream of exploring all the outer planets by the end of the 
1980s. Voyager and the skill, ingenuity, and resourcefulness of the 
people involved are about to make ninety percent of that dream come 

Bud Schurmeier 
Project Manager 

Although we realized that Voyager was embarking on a unique 
journey of exploration in 1977, none of us expected the wealth of 
discovery that followed. Voyager revealed new worlds in which 
familiar features appeared with unexpected diversity that challenged 
our understanding and expanded our view of the Solar System. 

Edward Stone 
Project Scientist 

In 1980 Voyager demonstrated elegantly that Saturn can be 
called the electrostatic giant planet. Its powerful and puzzling 
discharges are without parallel among the many exotic radio sources 
in space. How does Saturn energize its sources ? Voyager produced 
this surprising and wonderful question. Don't ignore it amid Voyager 

TIME: 0 Days. 4:45:00 
1989/08/25 8:45 GMT (SCT) 

Jim Warwick 
Principal Investigator 
Planetary Radio Astronomy 




49876 KM 

Those Known Widely 

Voyager — this modern Odysseus has uncovered the unimagined 
intricacy and awesome spectacle of the giant planets of our own home 
solar system. 

Neil Armstrong 
Astronaut and 
First Man on Moon 

Lots of science has come from Voyager. But more vital is that 
Voyager has shown us sights that had never before been seen , and 
thereby reminds us how much more remains unseen and unknown. 
It showed us how diverse worlds can be — and that's something every 
person on Earth should know. 

Richard Berry 
Astronomy Magazine 


We conquerors come 
But not to conquer 
Just to see , 

What you are now 
And what you used to be. 

We touch you with our sensor- 
Robot eyes , 

And wander swiftly down your 
alien skies, 

To weigh your body, face, your 
shape and size. 

Then turn to show the world 
Your wondrous parts, 

And so you'll conquered be. 

But in our hearts. 


Ray Bradbury 

Voyager is important , because to have the means to find out 
‘“what’s out there,” and not use it, is unthinkable. 

Angie Dickinson 

The Voyager spacecraft leave the solar system the greatest 
Earthborn explorers of all time. Even after their last radio link with 
Earth is severed, they will carry our technology, our spirit, and our 
music to the stars. The most awesome part of their mission is just 

Jon Lomberg 

Voyager 2 will leave the classic boundaries of our solar system in 
August of this year, leaving behind a twelve-year legacy of knowledge 
and continuing on as a consummate tribute to the imagination of its 
designers. As this fragile yet incredible speck of twentieth century 
technology sweeps on into interstellar space, it carries on board the 
precious cargo of human aspirations for the future. 

Syd Mead 

Through Voyager the human intellect has extended its horizons 
to the farthest reaches of the Solar System, initiating a new Age of 
Discovery, and giving mankind a deeper insight into the role of 
intelligence in the Universe. 

TIME: 0 Days, 4:50:00 

Thomas Paine 
NASA Administrator 




46842 KM 

The Voyager spacecraft have completed the preliminary 
reconnaissance of the planetary part of the solar system. They have 
examined for the first time dozens of new worlds, worlds our 
descendants will walk upon. That can only happen once in human 
history. It is our good fortune that it happened in our time, and that 
we were able to participate in this historic voyage of exploration and 

Voyager also represents, on its golden phonograph records, a 
hopeful — indeed, positively cheerful — message from the human species 
to other civilizations, if such exist, in the Milky Way galaxy. My wife, 
Ann Druyan, and I are especially happy to have helped design a 
message that will outlast our civilization and our species. 

Carl Sagan 

Professor of Astronomy 

Cornell University 

I have, indeed, been thrilled with the outstanding successes of the 
Voyager missions. 

Clyde Tombaugh 
Astronomer and 
Discoverer of Pluto 


TIME: 0 Days, 4:55:00 
1989/08/25 8:55 GMT (SCT) 




44279 KM 

Two roads diverged in a wood, and I— I took the one less 
traveled by, and that has made all the difference. 

Robert Frost 


The Voyager successes at Jupiter, Saturn, and Uranus would never 
have happened without the extra-special efforts of many dedicated people, 
and the same will be true for Voyager’s future at Neptune and beyond. 
During the Neptune encounter, there will be some 300 people directly sup- 
porting the Project, as well as many more around the world (see Chapter 3) 
that help us communicate with the two Voyagers. 

The purpose of this chapter is to identify several key people associated 
with the various Project functional areas. After all, if you were planning a 
tour of Europe, there would always be a number of questions not covered in 
your trip brochure. You would naturally consult one of the trip leaders for 
answers to your special questions. 

An effort has been made in Table 15-1 to identify the most basic 
functional areas and the appropriate cognizant personnel. It is always 
difficult to select a few specific individuals when so many people are associ- 
ated with Voyager, but a line must inevitably be drawn somewhere. When 
using Table 15-1, it is assumed that any cognizant person for a given 
functional area can answer questions for those subareas contained to the 
right of the given area. For those of you who wish to see the complete Project 
organization, Table 15-2 has been provided. 

During the actual encounter, the JPL Public Information Office will, of 
course, answer questions from outside people, or will generally refer these 
questions to one or more of the people listed in Table 15-1. 



Table 15-1. Key Voyager Personnel 



N. Haynes 
G. Textor 

C. Kohlhase 
R. Rudd 

D. Griffith 

Voyager Operations Functions 

J. Belcher 


L. Broadfoot 


B. Conrath 



D. Gumett 



T. Krimigis 


E. Stone 


A. Lane 


E. Miner 

N. Ness 


P. deVries 

B. Smith 


E. Stone 


L. Tyler 


J. Warwick 


Science Planning 

P. Doms 

and Operations 

D. Finnerty 


H. Mardemess 


J. Hall 

L. Miller 
K. Savary 
G. Hints: 


M. Deutsch 
K. Weld 


D. Gray 


Mission Control 

J. Nash 


J. Jones 

T. Adamski 
I Webb 

Tracking Network 

C. Finley 


C. Brower 

Control Center 

E. Campos 

Voyager Mission Planning 

C. Kohlhase, J. Gerschultz 

Voyager Data System Development 


M. Urban, R. Ellis, R. Otamura 


G. Spradlin, E. Kelly, R. Hill, S. Howard 

TIME: 0 Days, 5:00:00 



42273 KM 


Table 15-2. Voyager Project Organization 



Table 15-2. Voyager Project Organization (Continued) 










— (N. TOY) - SYS ENGR 
















































Table 15-2. Voyager Project Organization (Continued) 














































































































Table 15-2. Voyager Project Organization (Continued) 



TIME: 0 Days, 5:15:00 




40314 KM 

With more knowledge comes a deeper, more wonderful 
mystery, luring one on to penetrate deeper still. 

Richard Feynman 


When Voyager 2 slips past Neptune to begin its interstellar mission, 
NASA/JPL spacecraft will have visited every known planet in the solar 
system except Pluto. Although these past missions have rewarded us with 
extraordinary glimpses of the other worlds orbiting the Sun, in many cases 
what we have collected is precisely that — glimpses. To understand better 
what these missions have shown us or hinted at, we must go back with other 

Consider the most unforgettable moments of the Voyager Project, as it 
unlocked mysteries at Jupiter, Saturn, Uranus, and soon, Neptune. During 
the past twelve years we were astonished by erupting volcanoes on Io and 
complex, colorful eddies in Jupiter’s atmosphere; organic compounds at 
Titan and the haunting beauty of the braids in Saturn’s rings; and the 
cockeyed “prizefighter’s face” geography of the Uranian moon Miranda. As 
gratifying as all these experiences were, they were achieved while our 
spacecraft were flashing by at incredible speeds. They all point to many 
other questions that we would like to answer by returning for another, more 
thorough, look. 

The same is true of other planets in the solar system. Venus, for 
example, has been studied by more spacecraft than any other planet — eleven 
Soviet craft and nine from the United States. But the radar mapping 
instrument on our next Venus mission, Magellan, which was launched 
recently by the Space Shuttle on May 4, 1989, is ten times more powerful 
than similar mappers on any previous spacecraft. Magellan’s data will allow 
planetary scientists to distinguish between confusingly similar categories of 
geological formations on Venus and get a much better idea of how our closest 
neighbor in the solar system evolved. 

Complex revisit missions are also in the works for Jupiter and Saturn. 
Project Galileo — due for launch in fall 1989 — will place a heavily instru- 
mented probe into Jupiter’s atmosphere and a capable spacecraft in orbit 
about the planet; both accomplishments will be firsts at this body. The 
Cassini mission to Saturn has similar goals, except the probe will be dropped 
into the thick atmosphere of Saturn’s moon Titan, with hopes that it will 
reach that body’s unseen surface. 



Planets are not the only targets of upcoming missions at JPL. One 
spacecraft, Comet Rendezvous Asteroid Flyby, or CRAF, will expand on the 
“snapshot” data of international missions to Comet Halley in 1986 by 
meeting up with and traveling along with Comet Kopff for more than three 
years. Several NASA/JPL mission designs also include asteroid encounters 
for the first time, taking advantage of passes through the asteroid belt as the 
spacecraft speed toward their next planetary target. 

Voyager 2 at Neptune and the launches of two planetary craft in 1989 
provide an inspiring prelude for another long-awaited space event: launch of 
the Edwin P. Hubble Space Telescope, probably in early 1990. By placing 
this telescope in orbit above the distortions of the Earth’s atmosphere, we 
will be able to detect objects 1 00 times fainter than those visible from ground- 
based telescopes. The Hubble telescope is a complex project that has been 
coordinated between three NASA centers, private contractors, and several 
institutions. JPL built the telescope’s Wide-Field/Planetary Camera, one of 
its chief instruments. 

Among the roster of solar system targets we shouldn’t neglect is the 
Earth itself. Advanced satellites from the United States and other countries 
will be launched in the early 1990s to give us a much more comprehensive 
and detailed view of our planet’s climate systems. JPL is preparing 
instruments such as the NASA Scatterometer, a device for studying ocean 
winds, due to be launched on a Japanese satellite. Other JPL projects 
include Topex, a satellite which will map circulation of the world’s oceans, 
and one of the orbiting platforms for Eos, a major NASA Earth-observation 


Apart from such unmanned space projects, JPL also staffs an office 
near Washington, D.C., supporting development of NASA’s space station, 
Freedom. This is a manned laboratory, the components of which will begin 
to be put in orbit as early as the mid-1990s. 

As you follow launches in the years ahead, you will notice changes in 
the launch vehicle used. In addition to Magellan, the next two planetary 
missions involving the United States (Galileo and Ulysses) will be carried 
into Earth orbit by the Space Shuttle. After each spacecraft is released by 
the Shuttle, an upper-stage motor attached to the 
probe fires and sends the craft off to its eventual 
destination. Several missions, notably Galileo to 
Jupiter, have undergone many launch scenarios and 
upper-stage configuration changes before and after 
the Challenger accident in 1 986. Galileo in particular 
will now take a circuitous path to Jupiter, swinging by 


TIME: 0 Days, 5:20:00 



41123 KM 

the Earth twice and Venus once to pick up “gravity assist” energy to 
compensate for an upper-stage motor less powerful than originally planned. 

After those missions on the Shuttle, we will return to expendable 
rockets for planetary spacecraft launches. Most of the JPL missions will use 
Titan IV rockets, more powerful versions of the vehicles that launched the 
Voyagers. The joint U.S. -French Topex/Poseidon satellite will be orbited by 
the European-built Ariane rocket. 

The following pages summarize JPL space projects being developed or 
under study. At the end is a descriptive summary of mission concepts that 
are probably farther in the future — and that would take us literally out to the 
threshold of the stars. 

Notice: Information in this chapter was correct at the time of Guide publication. 



Comparing Venus and the Earth is the story of two seemingly near- 
identical twins that grew up to be improbably different. Although the closest 
planet to Earth in size and distance from the Sun, Venus has a surface 
temperature of up to 480°C (900°F) and a crushing atmosphere of carbon 

Venus' atmosphere shrouds surface details from us, so orbiting space- 
craft must use imaging radar to map the planet. Magellan's radar will 
achieve resolutions about 10 times better than that of Soviet Venera 
spacecraft and 100 times better than the United States' last planetary 
mission, Pioneer Venus, launched in 1978. The Magellan data will help 
scientists answer questions in diverse areas, from the mechanics of continen- 
tal drift on Earth to the cause of Venus' high temperatures, and perhaps 
even the sequence of events that gave birth to our own planet. 

Objectives Instruments 

★ Infer geological history of Venus ★ Radar imager 

★ Acquire global imaging and altitudes ★ Radar altimeter 

★ Map surface features and craters ★ Radiometer 

★ Search for evidence for volcanoes, plate ★ Radio science 
tectonics, ancient Seas and rivers 

★ Study greenhouse effect 

★ Map gravity field specifics 

★ Compare with terrestrial planets 

TIME: 0 Days. 525:00 




42624 KM 










Propulsion Module 



Rocket Engine 

— Thermal Control 

Solar Panel Drive 
and Cable Wrap 

Solar Panel 


Launch 5/4/89 


Jj Interplanetary Cruise 

| Venus Orbit Insertion 8/10/90 

In-orbit Checkout (18 Days) Q 

Project Start: 
October 1983 

i i ttaaar mapping 

1 1 243 Days 

End of Nominal Mission 4/28/91 1 

End of Project 10/28/91 J 



Extended Mission 4/29/91 to ... £ 

1 i 1 i 



1990 1991 



The environment is so complex at Jupiter, the largest planet of our solar 
system, that the giant and its satellites are like a miniature solar system of 
their own. We will be able to add many times over to what was learned from 
the Pioneers and Voyagers when Galileo arrives in 1995 for direct measure- 
ments of the Jovian atmosphere and an orbital mission of at least two years. 

With the exception of the Vikings that landed on Mars, Galileo is the 
most complex planetary spacecraft ever built. The Galileo orbiter has two 
sections, one which slowly spins and the other which does not. In this way 
it combines the best aspects of previous spacecraft: experiments which 
measure fields and particles are on the spinning segment to cancel out 
interference from the spacecraft’s electronics, while cameras and other 
instruments that need to be held steady are on the “despun” segment. Five 
months before reaching Jupiter, the orbiter will release an instrumented 
probe which will make a parachuted descent into the planet’s highly active 

On its circuitous route to Jupiter, Galileo wall gain energy from two 
gravity assists at Earth and one at Venus, performing science observations 
during those encounters, including some unique observations of Earth’s 
Moon. Along the way, Galileo will also encounter two asteroids, Gaspra and 
Ida, as it traverses the asteroid belt between Mars and Jupiter. 


★ Directly sample Jupiter's atmosphere 

★ Conduct long-term studies of atmos- 

★ Conduct close-up studies of Jovian 

★ Map structure and dynamics of mag- 

★ Map thermal properties of planet 

TIME: 0 Days, 5:30:00 




★ Four optical sensors 

★ Four fields and particles 

★ Dust detector 

★ Radio science 


★ Three chemical analysis 

★ Cloud detector 

★ Radiometer 

★ Lightning and energetic 
particle detector 

f . 



44748 KM 









Probe Relay Jupiter 
Antenna ^ Atmosphere 




Scan Platform, Containing: 

• Photopolarimeter Radiometer 

• Near-Infrared Mapping Spectrometer 

• Solid-State Imaging Camera 

• Ultraviolet Spectrometer 

JJ Launch 10/12/89 - 11/24/89 
| Venus Flyby 2/9/90 

| Earth Flyby 1 12/8/90 

I Gaspra Asteroid Flyby 10/29/91 
| Earth Flyby 2 12/8/92 

Project Start: I Ida Asteroid Flyby 8/28/93 

October 1977 ■ i 

| Probe Release 7/7/95 

Jupiter Orbit Insertion/Probe Relay 12/7/95 

Jupiter Tour 12/7/95 -10/7/97 | 

I I I I I 

1989 1990 1991 1992 1993 1994 1995 1996 1997 



Wide-Field/Planetary Camera — Hubble Space Telescope 

The difference between NASA’s Hubble Space Telescope and current 
ground-based optical telescopes can be compared to the difference between 
Galileo Galilei's first telescope and its predecessor, the human eye. 

This orbiting observatory will detect objects 100 times fainter than 
those visible from Earth telescopes, with about 10 times greater spatial 
resolution. It will extend our reach in the cosmos from a present limit of 
about 2 billion light-years to roughly 15 billion light-years — allowing us to 
look back in time nearly to the beginning of the universe. 

JPL's contribution to this project is the Wide-Field/Planetary Camera, 
one of the telescope's mam science instruments. This camera can operate in 
two modes: “wide-field” mode views large areas of sky, allowing scientists to 
plot the spatial relationships of distant objects such as galaxies and quasars; 
the “planetary” mode views a narrower field and is designed for studying 
objects within the solar system. 


★ Study cosmic evolution and distances 

★ Image stars and galaxies 

★ Conduct star and galaxy motion studies 

★ Map interstellar energy distribution 

★ Search for planets around Sun and 
other stars 

★ Image planetary atmospheres and 
surfaces, satellites, asteroids and 

Hubble Space Telescope 

★ Wide-Field/Planetary 

★ Faint-object spectro- 

★ High-resolution spectro- 

★ High-speed photometer 

★ Faint-object camera 

★ Fine-guidance sensors 

TIME: 0 Days, 5:35:00 


TO TRITON: 47413 KM 



Hubble Space Telescope 




Fold Mirrors 


Light Seal 
Camera Head 




Light Channel 



Registration Fitting-Point B 

Relay Optics 

Optical Bench 
(Graphite Epoxy) 







| Launch March 1990 
] Orbital Verification (2 months) 

□ Science Verification (6 months) 

1 Science Operations 


| Installation of WF/PC-2 3/94 

Science Observations £ 

Project Start: October 1977 


End of Mission 
2005+ ► 

J L 


1989 1990 1991 1992 1993 1994 1995 1996 1997 



Astronomers have learned over time that the Sun, a seemingly homoge- 
nous ball of light and heat, is in fact a complex realm of its own with diverse 
structural, thermodynamic, and nuclear phenomena. Until now we have 
only been able to study the plasmas and particles streaming from the Sun 
from a perspective within the ecliptic — the two-dimensional plane in which 
the Earth and most of the planets orbit the Sun. 

Ulysses, a joint mission between the European Space Agency (ESA) 
and NASA, will add a third dimension to this view by studying the Sun, solar 
wind and interstellar space at almost all solar latitudes. After a launch from 
the Space Shuttle, the ESA-developed Ulysses spacecraft will travel first to 
Jupiter, where the gravity of the giant planet will bend the spacecraft's path 
up and away from the ecliptic plane. The spacecraft will then travel back 
over the poles of the Sun and study it using its ESA- and NASA-supplied 
instruments for several years. 


★ Conduct fields and particles exploration 
of Sun's polar regions and regions far 
from ecliptic plane 

★ Characterize inner heliosphere at all 
solar latitudes 

TIME: 0 Days, 5:40:00 


★ Magnetometers (HED) 

★ Seven particle/wave/ 
plasma detectors 

- Solar-wind plasma 

- Solar-wind ions (GLG) 
— Low-energy ions and 

electrons (LAN) 

- Energetic particles 
and interstellar gas 

- Cosmic ray/solar 
particles (SIM) 

- Unified radio and 
plasma waves (STO) 

- Solar x-rays/cosmic 
gamma-ray bursts 

★ Dust detector (GRU) 

★ Radio science 

- Coronal sounding 

- Gravitational waves 




50533 KM 

Hidden in this 

| Launch 1 0/90 

| Jupiter Encounter 2/92 

I 1st Maximum Solar 
Latitude 8/94 

1 2nd Maximum Solar 
Latitude 6/95 

I End of Mission 

Project Start: October 1978 

1990 1991 1992 1993 1994 1995 



What causes the devastation of a climate phenomenon like the El Nino 
currents in the Pacific? Why do continents experience droughts one year and 
flooding another? To help answer these questions, scientists are collaborat- 
ing on an array of international experiments and studies in the 1990s to 
better understand interactions between the world’s oceans and long-term 
weather trends. 

NASA’s Ocean Topography Experiment, or TOPEX, and France’s 
Poseidon mission have been combined to make highly detailed and accurate 
maps of the marine geoid — the shapes produced by sea levels around the 
world. Sea level is related to ocean currents and eddies, and by taking 
gravity into account researchers will also be able to calculate major features 
of the ocean floors. The U.S. -built spacecraft will be launched by a French 
Ariane rocket and carry instruments contributed by each of the two coun- 
tries. Its primary mission is planned to span three years. 


★ Observe ocean topography for several 
years, supporting global studies of: 

- Ocean circulation and variability 

- Ocean dynamics and role in climate 

- Circulation/wind interactions 

- Current/wave interactions 

— Heat, mass, nutrient and salt 

- Tides 


★ Two altimeters 

★ Microwave radiometer 

★ Laser retroreflector 

★ Two positioning system 

TIME: 0 Days. 5:45:00 



54028 KM 




Solar Array 

Zenith Omni Antenna (Nadir 
Omni on Bottom- Not Visible) 

High Gain 
Antenna ^ 

Control Module 




Doris Antenna 
Laser Retroreflector Assembly 

Microwave Radiometer 
Instrument Module 
Altimeter Antenna 



| Launch 6/92 

Q Engineering Assessment (30 Days) 

□ Verification Phase (6 Months) 

| 1 Prime Mission (3 Years) 

| End Of Prime Mission Mid-1995 

Project Start: | | Extended 

October 1 986 Mission 

(Through 9/1/97) 

I 1 I I I I I 

1992 1993 1994 1995 1996 1997 




AND P’ ‘OTnooA.pH 


Mars Observer 

Much as Magellan at Venus will improve on the missions before it, 
Mars Observer will provide views of the red planet beyond those possible 
from the Viking orbiters launched in 1975. The spacecraft is the first in a 
series called Planetary Observer, which adapts the bus of a satellite typically 
used only for Earth- orbiting missions for use as a general inner solar system 
explorer. Mars Observer is scheduled for launch in 1992 with a Titan III 
rocket and Transfer Orbit Stage (TOS), a new upper stage concept. 

The mission’s chief purpose is to study the surface, atmosphere and 
climate of Mars throughout a full Martian year of 687 Earth days. Imaging 
from an orbit lower than that of the Viking orbiters, the Mars Observer 
Camera will produce panoramic, high-resolution surface maps — useful for 
planning future lander missions. The spacecraft will also contain French- 
built equipment to relay data from surface-exploration balloons released by 
the Soviet Union’s Mars ’94 mission. Ten Soviet scientists are directly 
participating in Mars Observer studies. 


★ Conduct global studies of: 

- Elemental composition of surface 
— Distribution of surface minerals 

- Topography 

— Gravitational field 

- Seasonal movement of water 
and dust 

- Atmospheric circulation 


★ Gamma ray spectrometer 

★ Laser altimeter 

★ Wide-angle, high- 
resolution camera 

★ Two thermal emission 

★ Magnetometer 

★ Mars '94 balloon data 

★ Radio science 

TIME: 0 Days, 5:50:00 




57833 KM 





Sensor Assembly 

Emission Spectrometer 

Mars Observer Camera 

Mars Horizon Sensor 

High Gain Antenna 

Solar Array 





Gamma Ray 

| Launch 9/10/92 

I I Interplanetary Cruise 

0 Mars Orbit Insertion 8/ 1 3/93 - 9/1 0/93 
□ Intermediate Orbit 

Project Start: 
October 1983 

| Mapping Orbit Established 12/6/93 

I I Operations 

12/16/93 - 9/6/95 

End of Mission 9/6/95 | 

1992 1993 1994 1995 


Comet Rendezvous Asteroid Flyby 

The more scientists have learned about comets in recent years, the 
more they have become intrigued. For years, it has been suspected that 
comets were very primitive objects from the outer solar system, essentially 
unchanged from the era in which the solar system formed, and that they may 
have originally brought water to the inner planets. Data retrieved from the 
international missions to Comet Halley in 1986 also suggested that comet 
nuclei contain organic compounds, which raises other questions about their 
role in the creation of life on Earth. 

Comet Rendezvous Asteroid Flyby, or CRAF, will extend the Halley 
experience by meeting Comet Kopff near the orbit of Jupiter and traveling 
along with it for at least three years as the comet loops around the Sun. It 
will also launch a penetrator-lander which will directly sample the comet’s 
nucleus. On the way to Kopff, CRAF will encounter the asteroid Hamburga. 
CRAF will be the first in a new series of outer planet exploration missions 
using the JPL-designed Mariner Mark II spacecraft bus. 


Comet Rendezvous 

★ Characterize comet nucleus and coma 

★ Study process of comet tail formation 

★ Study tail dynamics and interactions 
with radiation and solar wind 

Asteroid Flyby 

★ Characterize structure and geology 

★ Determine distribution of minerals, 
metals, and ices 

★ Measure mass, density, and nearby 


★ Comet penetrator/lander 
(includes temperature 
probes and surface 
strength and composition 

★ CCD narrow- and wide- 
angle cameras 

★ Near- and far-infrared 

★ Seven particle/dust/ice/ 
gas/plasma analyzers 

★ Magnetometer 

★ Radio science 

TIME: 0 Days, 5:55:00 



61889 KM 


I Launch 8/22/95 

| Earth Flyby 7/6/97 

| Hamburga Asteroid Flyby 1/22/98 


____ _] Operations 
Comet KopfT Arrival 8/14/00 

II Penetrator/Lander 

Delivery 7/20/01 

1 2/1 2/02 

October 1 989 (Proposed) End of Mission 3/31/03 1 

1995 1996 1997 1998 1999 2000 2001 2002 2003 



Cassini will follow CRAF as the second mission in the new Mariner 
Mark II series of spacecraft to the outer solar system. Named for the Franco- 
Italian astronomer who discovered the gap in Saturn’s rings (as well as 
several Saturnian moons), Cassini will return to the ringed planet for four 
years of orbital studies. 

A probe provided by the European Space Agency and carried to Saturn 
by the orbiter will descend to the surface of Titan, Saturn’s largest moon. 
Named Huygens, for the Dutch scientist who discovered Titan, the probe will 
study the atmosphere of the moon, which the Voyagers showed to have 
organic chemistry similar to simple precursors to life. If it survives the 
descent, the probe will continue to relay (for several minutes only) data from 
Titan’s surface, which may be covered with puddles or even oceans of liquid 



★ Six optical sensors 

★ Nine fields and particles 

★ Titan radar mapper 

★ Radio science 


★ Five atmospheric charac- 
terization instruments 

★ Descent imager and 

★ Radar altimeter 

★ Lightning and radio 
emission detector 

★ Surface science package 

Note: Above instruments from 
time: o Days. 6:00:00 model (not approved) payload. 


★ Conduct detailed studies of Saturn's 
atmosphere, rings, and magnetosphere 

★ Conduct close-up studies of Saturnian 

★ Characterize Titan's atmosphere and 




66152 KM 



I Huygens Probe Descent 
to Titan 3/17/03 

Project Start: 

October 1989 (Proposed) 

1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 

I End of 
I Mission 
j 12/31/06 


Earth Observing System 

The most ambitious study of our own planet will begin in late 1997 
when NASA launches the first orbiting platform in the Earth Observing 
System, or Eos. Combining instruments that study atmosphere, oceans, 
land surfaces and the solid Earth, the two initial Eos platforms built by 
NASA’s Goddard Space Flight Center (Eos-A and Eos-B) may be joined by 
two spacecraft from Europe and one from Japan. 

Both of the U.S. platforms will be launched into polar orbits on Titan 
IV rockets and are scheduled to be replaced every five years. The goal of the 
program is to study the entire Earth as a system, from the innermost core to 
the outermost magnetic field boundary. 

Although the definition of the Eos program is still changing, JPL will 
continue to be involved in the general coordination of the scientific objectives 
and the development of the instruments. 


★ Study Earth as a unified system, 

- Rainfall 

— Oceanography 

- Atmospheric composition 
and processes 

- Pollution and fires 

- Land-based phenomena 

— Distribution of ice and snow 

- Solar and magnetospheric 

★ Conduct high-resolution mapping 
of surface 

Instruments (JPL provided) 

★ Synthetic aperture radar 

★ Five atmospheric charac- 
terization instruments 

★ Three geoscience pack- 

★ Space environment 

TIME: 0 Days, 6:05:00 



70583 KM 



✓ Synthetic Aperture 
Radar Antenna 

Microwave Limb 
Sounder Radiometer 

Stratospheric Wind 
Infrared Limb Sounder 
Communications and Command 

Solar Array 
Reaction Wheels 



X-Ray Imaging Experiment 



Launch 12/97 


Mission Ops 
Replace 2002 

Mission Operations 

I Replace 2007 


f] Launch 12/99 

Mission Ops 1” \ 

Mission Ops |~~ ■ 


Replace 2004 

Mission Operations 

— i 

Replace 2009 [1 

uu l w 


Project Start: October 1990 

i i i i 

Mission Operations 

J I L 

1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 


Mars Rover Sample Return 

As the most ambitious exploration of Mars short of landing humans 
there, Mars Rover Sample Return would send a rover robot to roam across 
the planet, collecting soil and rock samples for return to Earth. 

The rover itself — prototypes of which have been under development at 
JPL for several years — will be equipped with stereo computer vision and 
sophisticated artificial-intelligence expert-system software to allow it to 
make many immediate maneuvering and science decisions itself. This is 
necessary because the round-trip speed-of-light time of up to 40 minutes 
from Earth to Mars precludes a ground controller commanding every rover 
action. The rover would be able to travel up to 100 km (62 mi) per year as it 
conducts its studies of the Martian surface. 

In one reference scenario, four rockets would separately launch an 
imaging orbiter, a communications orbiter, the rover lander, and a sample 
return vehicle. The returned samples would be retrieved in Earth orbit by 
the Space Shuttle. 

JPL is specifically studying the rover and sample return mission 
concepts . The four-mission concept shown in the schedule may be integrated 
into a much larger multinational effort involving many countries. 


★ Directly study Martian surface 

★ Return samples of surface and 
atmosphere to Earth for analysis 

★ Determine history, environment, 
and climate 

★ Demonstrate readiness for human 
exploration of Mars 



★ Stereo cameras 

★ Multispectral imagers 

★ Four geoscience packages 

Imaging Orbiter 

★ High-resolution camera 
(down to 1-m resolution) 

TIME: 0 Days, 6:10:00 


TO TRITON: 75154 KM 



High Gain Antenna 

Nuclear Power 

Instrumentation and 
Control Electronics 

Imaging | Launch 1996 
Orbiter I 

Stereo Cameras 

Robotic Sampling 

Drive System 

Communications | Launch 1998 
Orbiter | 

] Mission Operations 1996-2000 

Rover | Launch 1998 

j Operations 


Proposed Project Starts: 
October 1992, 1993, 
1994, and 1995 

Sample | Launch 2001 
Return j 

J Operations 

J Mission Ops 
| Earth Return 

i i 


1996 1997 1998 1999 2000 2001 2002 2003 2004 


Mission Concepts 

In addition to the current space projects with organized teams, many 
other mission concepts are under study at JPL. Some may be proposed as 
missions for the 1990s, while others are possible projects for the early 21st 
century. Examples include: 

Circumstellar Imaging Telescope: 

This telescope, attached to the U.S. Space Station Freedom, would 
search for protoplanetary material or even very large planets around nearby 

Large Deployable Reflector: 

This would be an array of telescopic mirrors in Earth orbit to study 
cosmology, galactic evolution, the interstellar medium, star formation, and 

Lunar Geoscience Observer: 

A follow-on mission in the Planetary Observer series, this craft would 
be placed into polar orbit around the Moon to measure its elemental and 
mineralogical composition and gravity, and assess potential resources for a 
manned lunar base. 

Comet Nucleus Sample Return: 

This follow-on mission in the Mariner Mark II series would acquire and 
return to Earth samples of a comet nucleus core, a sealed sample with 
volatile components, and a surface sample. 

Mercury Orb iter: 

Specially engineered for a hot environment, this craft would study 
Mercury’s magnetic origin, magnetosphere, atmosphere, ionosphere, sur- 
face, fields and particles, and solar physics. 

Thousand Astronomical Unit (TAU): 

Equipped with a nuclear reactor, ion propulsion, 

TIME: 0 Days, 6:15:00 . 

1989/08/25 10:15 gmt (sct) and light- wave communications system, this advanced 

spacecraft would be sent on a 50-year journey 1,000 
astronomical units (about 1 00 billion miles) into space. 
Initially TAU would be directed for an encounter with 
Pluto, followed by passage through the heliopause 
and perhaps the inner Oort Cloud, the hypothetical 


TO TRITON: 79841 KM 

region where comets originate. This vehicle would be instrumented to 
investigate low-energy cosmic rays, low-frequency radio waves, interstellar 
gases, gravity waves, and other deep-space phenomena. One of its chief uses 
would be to perform high-precision astrometry — measurements of the dis- 
tances between stars. 


TIME: 0 Days, 620:00 
1989/08/25 1020 GMT (SCT) 




84624 KM 

Every great advance in science has issued from a new 
audacity of imagination . 

John Dewey 


The Voyager mission has been in the making for over two decades, 
including the pre-Project work. Much has happened here on Earth during 
that time. Some of the Flight Team members have spent their entire working 
careers on this project. Many have started and raised their families in synch 
with mission events (how many people do you know who relate “Johnny was 
born between second Jupiter and first Saturn”?) We thought it might be fun 
to look back on the last 17 years on Earth, while the Voyager Project was in 
the making and making history: Although our list has a decidedly space- 
oriented bias, you might want to pencil in your own significant events. 
(Many thanks to Alexander Hellemans and Bryan Bunch, authors of The 
Timetables of Science, copyright 1988.) 


Congress approves Mariner 
Jupiter I Saturn ‘77 (MJS’77) 


Science payload for MJS’77 is 

NASA and Atomic Energy 
Commission sign agreement 
on nuclear power sources 
(RTGs) for MJS’77 

ERTS-1 (first Landsat) is launched 

First computerized axial tomography 
(CAT scan) is introduced 

Soviet Venera 8 soft lands on Venus 

Pioneer 10 is launched to Jupiter 

Apollo 1 7, last manned mission to the 
Moon, returns 110 kg of lunar material 

Mariner 10 is launched to Venus and 

U.K. joins European Common Market 

Pioneer 10, first spacecraft to encoun- 
ter Jupiter, experiences Jupiter's 
severe radiation environment 

A calf is produced from a frozen 

Three Skylab missions collect 171 
days of data 

First nuclear magnetic resonance 
(NMR) scanner is introduced 




Impact of Jupiter’s radiation Mariner 10 becomes first spacecraft to 
environment is studied encounter Mercury 

Standard trajectories are 

Seventy percent of spacecraft 
procurement contracts are 

U.S. President Richard M. Nixon 
resigns as a result of Watergate 
break-in cover-up 

Pioneer 11 encounters Jupiter and is 
redirected toward Saturn 

Skeleton of “Lucy,” 3-million year-old 
Australopithecus afarensis species, is 

Scientists warn that 
chlorofluorocarbons may be destroying 
Earth’s protective ozone layer 

Hewlett Packard introduces first 
programmable pocket calculator 


Final Spacecraft System Last American troops are withdrawn 

Design Review from Vietnam 

Mission Operations System Very Long Baseline Interferometry is 

functional requirements implemented 


Soviet Veneras 9 and 10 return first 
Deliveries of key parts pictures of surface of Venus 

impacted by nation-wide 

recession Apollo-Soyuz Test Project is launched 

First liquid-crystal displays for digital 
clocks and calculators are marketed 

TIME: 0 Days, 625:00 
1989/08/25 1025 GMT (SCT) 




89490 KM 


Proof Test Model spacecraft 
assembled and begins 
environmental tests 

Assembly of flight spacecraft 

Tentative flight path selected 

U.S. celebrates 200th anniversary of 
Declaration of Independence 

Vikings 1 and 2 land on Mars 

Scientists in an airborne observatory 
discover rings around Uranus 

First commercial company aimed at 
genetic engineering is established 

Supersonic Concorde begins regularly 
scheduled passenger service 


Mariner Jupiter / Saturn 1977 
is renamed Voyager 

Voyagers 1 and 2 are launched 

Voyager 1 returns first 
spacecraft photo of Earth and 

U.S. space shuttle Enterprise 
completes first approach and landing 

The King of Rock ‘ri Roll, Elvis 
Presley, dies 

Two men in New York become earliest 
victims of Acquired Immune 
Deficiency Syndome (AIDS) 

Last recorded case of smallpox in the 
wild is found in Somalia 

Balloon angioplasty is invented to 
unclog diseased arteries 

Apple II computer is introduced 


Voyager 2’ s main radio 
receiver fails; tracking loop 
capacitor fails in backup 

Backup mission load 
designed to perform Jupiter 
encounter in event of receiver 

Charon, only known satellite of Pluto, 
is discovered 

Seasat performs imaging radar studies 
of ocean's surface from space 

Pioneer Venus probes are launched 

First “test-tube baby” is born 

Planning begins for Jupiter 



As Voyager 1 swings through 
the Jovian system, it dis- 
covers eight active volcanoes 
on Io and a thin dust ring 
around Jupiter 

Four months later, Voyager 2 
is able to observe six of Io’s 
volcanoes still erupting 

Planning begins for Saturn ob- 

Egypt and Israel sign accord at Camp 
David to end their 30-year war 

Human-powered airplane Gossamer 
Albatross crosses the English channel 

Nuclear reactor at Three Mile Island, 
NJ undergoes a partial meltdown 

Pioneer 11 becomes first spacecraft to 
encounter Saturn 


Voyager 1 swings through the 
Saturn system, reveals 
complexity of Saturn’s rings 
and sees a haze-enshrouded 

Voyager 1 begins its trip out of 
the solar system, rising above 
the ecliptic plane 

Voyager Project recommends 
that Voyager 2 be sent on to 

Very Large Array begins operation in 
Socorro, NM 

Mt. St. Helens erupts 

Successful production of human 
interferon in bacteria is announced 

Machine is developed to break up 
kidney stones with sound waves 

Scientists postulate giant body 
impacted Earth and contributed to 
extinction of dinosaurs 

TIME: 0 Days, 6:30:00 
1989/08/25 10:30 GMT (SCT) 




94425 KM 


Voyager 2 swings through 
Saturn's system and heads for 

Scan platform on Voyager 2 
sticks 1 02 minutes after 
closest approach to Saturn 

Chinese scientists successfully clone a 

Black-footed ferrets, thought extinct, 
are found in Wyoming 

Genetic code for hepatitis B surface 
antigen is found 

Solar One, world’s largest solar-power 
station, is completed near Barstow, 

IBM personal computer, using DOS, is 

First U.S. space shuttle, Columbia , 
completes first mission 


Tasks are defined to compress 
image data, to use FDS 
processors in tandem, and to 
use Reed-Solomon encoding 

Deep Space Network upgrades 
two 26-m antennas to 34-m 

Mary Rose , a 16th-century warship, is 
raised from Portsmouth, England 

Soviet Venera 13 and 14 soft land on 

Single atom of element 109 is created 

El Chicon volcano erupts in Mexico, 
sending dust and gases into 

First Jarvik-7 artificial heart is 

Compact disc players are introduced 



Ground testing of scan 
platform flight spare actuators 

Infrared Astronomical Satellite (IRAS) 
discovers evidence of planet formation 
around stars outside our solar system 

Implementation of new flight Immunosuppressant cyclosporine is 
software begins approved for use in U.S. for organ 


Detailed planning for Uranus 


Campaign to reduce image 
smear begins 

Soviet researchers drill to a depth of 
12 km and reach Earth's lower crust 

Work continues to dev:ise Los Angeles hosts the XXIIIrd 

strategy for use of ailing scan Olympiad 

platform on Voyager 2 

Apple Computer introduces the 
Preliminary planning begins Macintosh 

for Neptune encounter 

Genetic fingerprinting technique is 

First successful surgery is performed 
on a fetus before birth 


Implementation of new flight 
software is completed 

North polar region is selected 
for aimpoint at Neptune 

Work begins on array of 
Canberra antennas with 
Parkes Radio Observatory 

TIME: 0 Days. 6:35:00 
1989/08/25 10:35 GMT (SCT) 

Volcano Nevada del Ruiz erupts in 
Columbia, killing 25,000 people 

British Antarctic Survey detects 
spring-time hole in ozone layer over 

Lasers are used to clean clogged 

Soviet Vega 1 and 2 drop landers on 
Venus enroute to Halley’s Comet 




99420 KM 


Voyager 2 becomes the first 
spacecraft to swing by Uranus, 
discovering ten new moons, 

Voyager aircraft circles the globe in 
nine days without refueling 

two new rings, and an unusual 
magnetic field 

U.S. space shuttle Challenger 
explodes, killing 7 astronauts 

Deep Space Network begins 
upgrades including expansion 
of 64-m antennas to 70-m 

International armada of spacecraft 
encounter Halley’s Comet 

Neptune aimpoint reconfirmed 

Chernobyl nuclear reactor explodes 
near Kiev, U.S.S.R 

Detailed planning begins for 
Neptune encounter 


Periodically inhabited Soviet space 
station Mir is launched 

Voyager 2 “observes” 
Supernova 1987A 

Last wild California condor is trapped 
and placed in zoo for breeding 

Deep Space Network 
completes 34-m high efficiency 
antenna at Madrid 

IBM introduces Personal System/2 

Work begins to gain additional 

Supernova 1987A is observed 

radio science coverage with 
Usuda Observatory, Japan 

Yuri V. Romanenko sets new record 
for days in space, 326 days 

Nodding image motion 
compensation is tested 

Last dusky seaside sparrow dies in 

U.S. New York Stock Exchange drops 
508 points (more than 22 percent) in 
one day 



New array of Golds tone with 
Very Large Array is tested 

Voyager 2 continues intensive 
stellar UV astronomy 

FDS dual processor program 

Scientists directly observe the 
atmosphere of Pluto 

First U.S. patent is issued for a 
vertebrate product of genetic 
engineering (a white mouse) 

Apple introduces Macintosh II 

Voyager 2 returns first color Computer parallel processing speeds 

images of Neptune solutions by a factor of 1000 

Human-powered airplane Daedalus 88 
sets new distance and time records 

First Soviet space shuttle is launched 
unmanned and recovered 

U.S. resumes space shuttle launches 


Manueverless image motion 
compensation tested 

Voyager 2 will become first 
spacecraft to swing through 
Neptune's system, hoping to 
learn more about Neptune's 
atmosphere, Triton, and 
purported ring arcs 

Voyager 2 will begin its trip 
out of the solar system, below 
the ecliptic plane 

Scientists discover disturbed 
chemistry related to chlorofluoro- 
carbons in stratosphere over Earth’s 
high northern latitudes 

Pluto reaches its closest point to the 
Sun in its 284-year orbit 

Scientists claim to have demonstrated 
table-top, sustained, net-energy- 
releasing nuclear fusion 

Supertanker spills millions of gallons of 
oil into Alaska's Prince William Sound 

TIME: 0 Days. 6:40:00 
1989/08/26 10:40 GMT (SCT) 

Trans-Pacific communications cable 
begins operation 

U.S. Magellan spacecraft is launched to 

Speaker of the House resigns 

U.S. Galileo spacecraft is scheduled for 
launch to Jupiter 



104466 KM 


1 Stars scribble in our eyes the frosty sagas , 

the gleaming cantos of unvanquished space . 


Hart Crane 


We’ve tried in this Guide to pass along the excitement, wonder, and 
challenges of the Voyager missions, but still keep the explanations fairly 
simple. We hope we’ve whet your appetite to learn more. Much more has 
been written about Voyager, both in the popular press and in the scientific 
literature. We present here a brief list of further reading, if you so desire, for 
your spare time. Some of you may enjoy just browsing through one of the 
popular books, looking at nifty pictures, whenever a few idle minutes are 
available. Others of you may want to dive in and dig for the nitty-gritty 
details of the engineering and science of the mission. Whatever your choice, 

Books on Voyager: 

Cooper, Henry S. F. Imaging Saturn: The Voyager Flights to Saturn. Owl 
Books, H. Holt & Co., 1983. 

Davis, Joel. Flyby , the Interplanetary Odyssey of Voyager 2. New York: 
Atheneum, 1987. 

Morrison, David C. and Jane Samz. Voyage to Jupiter. NASA SP-439. 
Washington, D.C: National Aeronautics and Space Administration, 1980. 

Morrison, David C. Voyagers to Saturn. NASA SP-451. Washington, D.C. : 
National Aeronautics and Space Administration, 1982. 

Poynter, Margaret and Arthur L. Lane. Voyager: The Story of a Space 
Mission. New York: Atheneum Childrens Books, Macmillan, 1981. 

Sagan, Carl, and F. D. Drake, Ann Druyan, Timothy Ferris, Jon Lomberg, 
and Linda Salzman Sagan. Murmurs of Earth, the Voyager Interstellar 
Reco?'d. New York: Random House, 1978. 

Washburn, Mark. Distant Encounters. Washington, D.C.: Harcourt, Brace, 
Jovanovich, 1983. 

Note: Listings here are for information only and do not necessarily imply endorsement. 


Books Pertaining to Neptune: 

Burgess, Eric. Uranus and Neptune . The Distant Giants. Columbia 
University Press, 1988. 

Grosser, Morton. The Discovery of Neptune. Cambridge: Harvard Univer- 
sity Press, 1962. 

General Books on the Solar System: 

Beatty, J. Kelly, Brian O’Leary, and Andrew Chaikin, eds. The New Solar 
System. Cambridge: Cambridge University Press and Sky Publishing 
Corporation, 1981. 

Frazier, Kendrick and The Editors of Time-Life Books. Solar System. Planet 
Earth series. Alexandria, VA: Time-Life Books, Inc., 1985. 

French, Bevan M., and Stephen P. Maran, eds. A Meeting with the Universe. 
NASA EP-177. Washington, D.C.: National Aeronautics and Space 

Administration, 1981. 

Gallant, Roy A. National Geographic Picture Atlas of Our Universe. 
Washington, D.C.: National Geographic Society, 1980. 

Hoyt, William Graves. Planets X and Pluto. Tucson: University of Arizona 
Press, 1980. 

Littman, Mark. Planets Beyond, Discovering the Outer Solar System. Wiley 
Science Editions. New York: John Wiley & Sons, Inc., 1988. 

Smoluchowski, Roman. The Solar System. Scientific American Library. 
New York: Scientific American Books, 1983. 

The Editors of Time-Life Books. The Far Planets. Voyage Through the 
Universe series. Alexandria, VA: Time-Life Books, Inc., 1989. 

Trefil, James S. Space Time Infinity. New York: Pantheon Books and 
Washington, D.C.: Smithsonian Books, 1985. 

TIME: 0 Days. 6:45:00 
1989/08/25 10:45 GMT (SCT) 




109557 KM 


Magazine Articles : 

Popular Articles : 

Articles on Voyager often appear in popular magazines such as Astronomy, 
Discover, Mercury, National Geographic, New Frontier, Odyssey, Omni, Sci- 
ence News, Sky and Telescope, and The Planetary Report. Some specific 
articles of interest are listed below. 


Voyager and Planets: 

Gore, Rick, “VoyagerViews Jupiter’s Dazzling Realm.” National Geographic, 
January, 1980. 

Gore, Rick, “Saturn, Riddles of the Rings.” National Geographic, July 1981. 

Gore, Rick, “Between Fire and Ice, The Planets.” National Geographic, 
January 1985. 

Gore, Rick, “Uranus, Visit to a Dark Planet.” National Geographic , August 

Laeser, Richard P., William I. McLaughlin, and Donna M. Wolff, “Engineer- 
ing Voyager 2’s Encounter with Uranus.” Scientific American , November 

Wiley, John P., Jr., “A spacecraft named Voyager has shown us the moons 
and rings of Uranus — and just how well a machine can perform.” Smith- 
sonian, September 1986. 

Wilkinson, Stephan, “Space Geniuses Wanted: Apply JPL.” Air & Space / 
Smithsonian, December 1986/January 1987. 

Neptune Encounter: 

Beatty, Kelly. “Voyage to a Far Moon.” Omni, December 1988. 

Berry, Richard. “Voyager’s first glimpse of Neptune: a year before it 

encounters Neptune, Voyager 2 took its first tantalizing images of Neptune 
and its moon Triton.” Astronomy, October 1988. 

Berry, Richard. “Triton.” Astronomy, February 1989. 

Chiles, James R. “To the Stars!” Reader s Digest , December 1988. 

Eberhart, Jonathan. “Planetary perks: scientific fringe-benefits of Voyager 
2’s trip to Neptune.” Science News, September 10, 1988. 



Eberhart, Jonathan. “High expectations for Voyager 2 at Neptune.” 
Science News , November 12, 1988. 

Kohlhase, Charles. “On Course for Neptune.” Astronomy, November 1986. 
Kohlhase, Charles. “Aiming for Neptune.” Astronomy, November 1987. 
Lemonick, Michael. “Neptune's Baffling Clouds.” Science Digest , January 

Littman, Mark. “The Triumphant Grand Tour of Voyager 2.” Astronomy , 
December 1988. 

Miner, Ellis D. “On to Neptune.” The Planetary Report, November/December 

O'Meara, Stephen J. “Neptune Through the Eyepiece.” Sky and Telescope , 
May 1989. 

Sohus, Anita, and Ellis Miner. “The Voyager Mission to Neptune.” Mercury 
(magazine of the Astronomical Society of the Pacific), September/October 

Stevenson, David J. “Looking Ahead to Neptune.” Sky and Telescope , May 

Thorpe, Andrew M. “Enigmatic Triton and Nereid." Sky and Telescope , May 

Scientific Articles : 

Voyager science results are published in professional magazines such as 
Geophysical Research Letters , Icarus , Journal of Geophysical Research , 
Nature , Science , Scientific American, and Space Science Reviews, as well as 
in the proceedings of conferences of professional societies such as the 
American Astronautical Society (AAS), the American Geophysical Union 
(AGU), and the American Institute of Aeronautics and Astronautics (AIAA). 



Space Science Reviews 21, (no. 2, November 1977 and 
no 3, December 1977) 



114686 KM 

Official results of the first analyses of the Voyager encounter data are 
traditionally published in Science , the magazine of the American Associa- 
tion for the Advancement of Science: 

Science 204 (June 1,1979): pp. 913-924 and 945-1008 (Voyagerl at Jupiter) 
Science 206 (November 23, 1979): pp. 925-996 (Voyager 2 at Jupiter) 
Science 212 (April 10, 1981): pp. 159-243 (Voyager 1 at Saturn) 

Science 215 (January 29, 1982): pp. 459 and 499-594 (Voyager 2 at Saturn) 
Science 233 (July 4, 1986): pp. 1-132 (Voyager 2 at Uranus) 

Bergstrahl, Jay, ed. “Uranus and Neptune,” NASA Conference Publication 
2330. Washington, D.C.: NASA, 1984. 

Hubbard, W. B. “1981N1: A Neptune arc? (or new satellite).” Science 231 (14 
March 1986): 1276. 

Hunten, Donald M. and David Morrison, eds. “The Saturn System,” NASA 
Conference Publication 2068. Washington, D.C.: NASA, 1978. 

“Voyager Missions to Jupiter.” reprinted from Journal of Geophysical Re- 
search 86 (September 30, 1981, no. A10). 

General Interest: 

Brown, Robert Hamilton, and Dale P. Cruikshank. “The Moons of Uranus, 
Neptune, and Pluto.” Scientific American 253 (July 1985): 38. 

Drake, Stillman, and Charles E. Kowal. “Galileo's sighting of Neptune.” 
Scientific American 243 (December 1980): 74. 


TIME*. 0 Days, 6:55:00 
1989/08/25 10:55 GMT (SCT) 

>5 GMT (SCT) 



119849 KM 

Here lyeth muche rychnesse in lytell space . 

John Heywood 


An acronym is a handy type of shorthand, a word formed from the 
first letters of the words that make up a lengthy name or phrase. Some 
acronyms have attained the status of becoming “real” words; for example, 
radar (radio detection and ranging) or snafu (situation normal, all fouled up). 
.Although used by some of the great Medieval Jewish scholars to shorten 
their names (e.g., Rashi for Rabbi Shelomo ben Yitzhak, 1040-1105), most 
acronyms were coined during and after World War II. In fact, acronyms were 
introduced in the February 1943 issue of American Notes and Queries. 

The origin of the word is Greek ( akros [for top] plus onyrna [for 
name]), but did you know that “acronym” is also an acronym? Some clever 
souls just had to invent a suitable phrase to give it acronym status. If you 
like the sophisticated, then try Algorithm for Character Reconstruction of 
Names Yielding Mnemonics. If you want to be a bit more honest about it, 
then try A Contrived Reduction of Names Yielding Mnemonics. 

The very best acronyms become real words that have a meaning 
closely related to the acronym's own special significance. A Voyager classic 
is SAMPLER, standing for Science and Mission Profile Leaving Earth 
Region, a flight software sequence developed (but never executed as in- 
tended) to test and “sample” certain spacecraft and science capabilities 
shortly after launch. 

On the more humorous side, Gentry Lee, formerly Project Engineer 
for the Galileo project, once remarked that a project is ready to launch when 
the depth of its acronyms has reached a certain level. 

Virtually every walk of life develops its own set of acronyms. To the 
uninitiated, our conversations sometimes sound like alphabet soup, but 
acronyms can sure save time and typing! Sometimes, those of us who have 
been in this business for a long time still chuckle when we hear an acronym- 
rich dialog such as, “Our NET RFAs aren't due to low SNR at the DSN or 
VLA, but TBD problems may exist in the PLS EDRs, so the SOC will recheck 
the DACS for data gaps from CCS load B825.” The secret decoding guide 


* r ,f 



Voyager 1 reference 


Attitude and Articulation Control Subsystem 


All Axes Inertial 


anno Domini 


American Institute of Aeronautics and Astronautics 


Amplitude Modulation 


PRA investigation of Neptune's magnetosphere 


Astronomical Unit (approx. 150 million km or 
93 million mi) 

A z 



Voyager 2 reference 


Backup Mission Load 


bits per second 


Closest Approach 

C 3 

Command, Communications, and Control 


cubic centimeter 


Computer Command Subsystem 


Computer Command Subsystem Load 


Canberra Deep Space Communications Complex 


Capability Demonstration Test 


Command Detector Unit 




Command Moratorium 


Complex Monitor Control 




Cosmic Rays 


Cosmic Ray Subsystem 


Cruise Manuever 


Commonwealth Scientific and Industrial Research Or- 
ganization (Australia) 


TIME: 0 Days, 7:00:00 

Canopus Star Tracker 

1989/08/25 1 1 DO GMT (SCT] 





125043 KM 

j d 


1 DRS 



Data Capture and Staging 

Data Records Subsystem 

Deep Space Communications Complex 

Deep Space Network (NASA) 

Data Storage Subsystem, Deep Space Station 
Digital Tape Recorder 





Experiment Data Record 

Encounter-Relative (time) 

Earth-Received Time (occasionally, encounter-relative 



Extreme Ultraviolet 
Extreme Ultraviolet Explorer 










Fields and Particles 
Flight Data Subsystem 
Far Encounter Phase 
Frequency Modulation 
Flight Operations Office 
Field ofView 

Fault Protection Algorithm 
Flight Science Office 

LECP observation to detect rapid variations in Neptune's 
radiation field 




Far Ultraviolet 




Ground Communications F acility 
Ground Data System 

Goldstone Deep Space Communications Complex (Califor- 


GigaHertz (one billion cycles per second) 






Greenwich Mean Time 
General Science and Engineering 
Goddard Space Flight Center 


H Hydrogen 

h or hr hour 

He Helium 

HGA High Gain Antenna 












International Comet Explorer 
Image data compression 
Image Motion Compensation 

National Institute for Aerospace Techniques (Spain) 

Interplanetary Medium 


Infrared Interferometer Spectrometer and Radiometer 

Institute of Astronautical Science (Japan) 

Interstellar Medium 
Imaging Science Subsystem 
International Ultraviolet Explorer 


Jet Propulsion Laboratory 




kilo (1000) bits per second 



LAN Local Area Network 

lb pound 

LECP Low Energy Charged Particles Subsystem 

LEMPA Low Energy Magnetospheric Particle Analyzer (in LECP) 

LEPT Low Energy Particle Telescope (LECP) 

LETS Low Energy Telescope System (CRS) 

LEU Late Ephemeris Update 

LMC Link Monitor Control 

LSU Late Stored Update 

TIME: 0 Days, 7:05:00 
1989/08/25 1 1 :05 GMT (SCT) 

m meter (also, minute) 

MAG Magnetometer Subsystem 
MAGROLL Spacecraft roll maneuver 
MCCC Mission Control and Computing Center 
MCT Mission Control Team 




130264 KM 


MegaHertz (one million cycles per second) 






Maneuverless Image Motion Compensation 
Multimission Image Processing Subsystem 
Mariner Jupiter/Saturn 1977 




miles per hour 
Mission Planning Office 











Narrow Angle (imaging) 

National Aeronautics and Space Administration 

NASA Communications Network 

Navigation Team 

Near Encounter Phase 

Near Encounter Contingency 

Near Encounter Test 

Nodding Image Motion Compensation 

Neptune Movable Block 

National Radio Astronomy Observatory 







Orbiting Astronomical Observatory 
Observatory Phase 
Optical Navigation 

Operational Readiness Test (radio science) 
One-Way Light Time 











Pacific Daylight Time 
Post-Encounter Phase 
Public Education Office (JPL) 

Public Information Office (JPL) 

Plasma Subsystem 

Photopolarimeter Subsystem 
Photometric observations of Neptune’s atmosphere 
Planetary Radio Astronomy Subsystem 
Plasma Wave Subsystem 















S/C 31 
S/C 32 




sec, s 









TIME: 0 Days, 7:10:00 

The R-S-T axes refer to an orthogonal targeting coordi- 
nate system, where S points in the same direction as the 
target-relative approach hyperbolic excess velocity, T is 
parallel to the ecliptic plane, and R is "down" 

100 ergs per gram of irradiated material 

Request for Action 

Radio Frequency Subsystem 

Radio Oecultation Data Analysis 

IRIS observation of Nepune’s disk 

revolutions per minute 

IRIS observation of radio science oecultation point in 

Neptune’s atmosphere 

Reed-Sol omon; Radio Science 

Radio Science Subsystem 

Radioisotope Thermoelectric Generator 

IRIS map of lit side of Triton 

IRIS map of dark side of Triton 

Voyager 1 reference 
Voyager 2 reference 
Scan Converter 
Spacecraft Event Time 

Spacecraft Team (sometimes, Spacecraft Time) 

Science Data Team 

Supplementary Experiment Data Record 
Sequence Team 

Science I nvestigation Support Team 
Signal-to-Noise Ratio 
Science Operations Coordinator 
Signal Processing Center 
Science Steering Group 
Science Working Group 

^ r * v is Refer to R-axis 

To Be Determined 
Thermal Cycle 

Trajectory Correction Maneuver 
Electron Telescope (CRS) 





135509 KM 


























VRMOS1, 2 






Triton Movable Block 

Torque Margin Test 

Test and Telemetry Subsystem 



UVS observations of Neptune’s corona 
UVS observations of Neptune’s dark pole 
Ultra Stable Oscillator 

UVS observation of co-rotating plasma near Triton 
UVS observation of airglow emissions from Triton’s 

UVS observation of Triton’s outer atmosphere during 

UVS observation of co-rotating plasma near Triton 

Ultraviolet Spectrometer Subsystem 

Video Cassette Recorder 

Voyager Interstellar Mission 
Voyager Imaging Support Activity 
Very Large Array 
Vernier Movable Block 
Highest resolution picture of Nereid 
Voyager Neptune Encounter Science Support Activity 
Narrow-angle images of possible ring arcs 
Narrow-angle mosaics to search for possible ring arcs 
Images of possible ring arcs as sunlight is scattered 
through them 

Narrow-angle images to observe possible ring arcs or 
shepherding satellites 

Retargettable images of a possible newly discovered ring- 
arc (acronym subsequently changed to VRRETINX) 

See VRRET1; x = 0, 1, 2, 3 for specific application of 

Imaging observations during outbound ring plane cross- 

Highest resolution color images of Triton 
Highest resolution images of Triton 















Periodic imaging of Triton as it orbits Neptune 
Images to map Triton's surface 

Wide Angle (imaging) 

PWS observations of plasma wave signals near Neptune's 
north pole 
Work Station 

PWS observations of plasma wave signals in Neptune's 
inner magnetosphere 

5-second PWS observations of plasma wave signals in 
Neptune’s inner magnetosphere 

10-second PWS observations of plasma wave signals in 
Neptune’s inner magnetosphere 
World War II 

Radio science observations of rings during occultation 
Radio science observations of Neptune’s atmosphere 
during occupations 

Radio science observations of gravity fields of Neptune 
and Triton 

Radio science observations of Triton’s atmosphere during 

TIME: ODays, 7:15:00 



140776 KM 

Something hidden. Go and find it. Go and look 
behind the ranges. Something lost behind the 
ranges. Lost and waiting for you. Go! 

Rudyard Kipling 

20. INDEX 

Note: Detailed sub-indices appear in this index for the following 

Deep Space Network (DSN) 

Science Instruments 

Voyager Project 

Voyager Spacecraft 

Jupiter, Saturn, Uranus and Neptune 

Accuracy 1,22,23,137 

Achernar (star) 34 

Acronyms 258-266 

Adams, John Couch 9-11 

Aiming Point 64-72 

Alkaid (star) 34 

Astronomical Unit (AU) 143 

Atmospheres See planet desired 

Back-up Mission Load (BML) 40,88-89,102 

Bessel, Friedrich Wilhelm 9 

Beta Canis Majoris (star) 98 

Canopus (star) 34 

Cassini (space mission) 116,236-237 

Comet Rendezvous Asteroid Flyby (space mission) 220,234-235 

Computer Command Subsystem (CCS) Loads 20,24,41,72-74,80- 


Commanding 26,40-41 

Command Moratorium 40 

Communications See Telecommuni- 



Constants 70-72,89 

Contingency Planning 109-121 

Contingency Sequences 73,119 

Cruise Science 198-201 

Cruise Science Maneuver (CRSMVR) 84 

d’ Arrest, Heinrich 12-13 

Data Records 30,80 

Data Return 109,135 

Deep Space Network (DSN) 25-28,78-80, 

(See also Parkes, Usucla, Very Large Array) 120-121,132,136 

Arraying 132 

Complex Monitor Control (CMC) 132 

Deep Space Communications Complex (DSCC) 26-28,79,132 

Link Monitor Control (LMC) 132-133 

Signal Processing Center (SPC) 132 

Distance (or Range) 1,2,13,64,144,145, 


Doppler Shift 1,2,52-53,91 

Earth Base 1,2,28 

Earth Observing System (Eos, space mission) 220,238-239 

Ecliptic Plane 65-66,144 

Encounter Event Times (Table) 148 

Encounter Phases 63,84 

Environmental Hazards 69-71,113-116,137 

Ephemeris 22,162 

Exposure Times 46 

Failure (or Fault) Protection Algorithm (FPA) 

Far Encounter Phase 

Fields and Particles 

Flandro, Gary 

Flight Engineering Office 


Flight Operations Office 


Flight Science Office (FSO) 
Flight Team 











146063 KM 


Galileo (space mission) 108,219-220 

General Relativity, Test of 53 

Grand Tour 64,68,104-105,143- 


Gravity Assist Concept 103-108,139,146 

Ground Communications Facility (GCF) 79-80 

Ground Data System (GDS) 29,78-80 

Ground Data Systems Engineering Office (GDSEO) ....214 

Heliopause 145,151-153,161 

Hubble Space Telescope (space mission) 220,226-227 

Hydrazine 155 

Image Data Compression (IDC) 42,126-127 

Image Motion Compensation (IMC) 128,130 

Internal Structure, Planet See desired planet 

Jet Propulsion Laboratory (JPL) 21-30,78-80 

Future Space Missions 219-244 

Jupiter System 169-178 

Atmosphere 171-173 

Internal Structure 169 

Magnetosphere 178 

Rings 173-174 

Satellites 4,140,174-178 

Kuiper, Gerard 14-15 

Lassell, William 14 

Late Activities 74-77,88-90,100 

Late Ephemeris Update (LEU) 74-75,90,100 

Late Stored Update (LSU) 75,90,94-95,100 

Le Verrier, Urbain Jean Joseph 11-12 

Light Travel Time 145 

Magellan (space mission) 






See desired planet 

Maneuvers, Spacecraft 34-35 

(See also Trajectory Change Maneuvers) 

Maneuverless Image Motion Compensation (MIMC) ...97,129-130 

Mars Observer (space mission) 232-233 

Mars Rover Sample Return (MRSR, space mission) 240-241 

Media Coverage (of encounter) 145, Hip Pocket 

Milky Way Galaxy 66,143,162,167, 


Mission Control Team (MCT) 217 

Mission Planning Office (MPO) 23,24,213 

Movable Block 94-95,98 

Multimission Control and Computing Center 

(MCCC) 25,79-80 

Multimission Image Processing Subsystem (MIPS) 30,80 

NASA Communications Network (NASCOM) 78-80 

Navigation 1,2,71-72,85-86, 


Navigation Team 21,216 

Near Encounter Test (NET) 78 

Near Encounter Phase 63,90-1 00 

Neptune System 4,13-14,65-66, 


Atmosphere 1 3-1 4,1 1 6 

Discovery 9-12 

Internal Structure 13-14 

Magnetosphere 14,115-116 

Rings 17,113-115 

Satellites 13-18 

Nereid 5,13-14,17,22, 


Nodding Image Motion Compensation (NIMC) 128-130 

TIME: 0 Days, 7:25:00 

Observatory Phase 


Earth (Radio Science) 


Star (PPS) 

Oort Cloud 










151367 KM 

Operational Readiness Test (ORT) 77-78,88 

Optical Navigation 1,47 

Parkes Radio Telescope 27,124 

Pasadena 28, Hip Pocket 

Pioneer Spacecraft 25,105,143- 


Planet- ”X” 153 

Pluto 15,68-69,107-108, 


Post Encounter Phase 6 3,100-102 

Public Information Office (PIO) 211 

Radiation 14 

References 253-258 

Resolution 140 

Rings See desired planet 

Satellites See desired planet 

Saturn System 179-186 

Atmosphere 180 

Internal Structure 179 

Magnetosphere 186 

Rings 3,140-141,180-183 

Satellites 4,141,183-186 

Science 1,80-102,140-141 

Science Instruments 43,45-51 

Calibration 56,58 

Cosmic Ray Subsystem (CRS) 55-57 

Imaging Science Subsystem (ISS) 45-47,57 

Infrared Interferometer Spectrometer 

and Radiometer (IRIS) 47-48,57 

Low Energy Charged Particle Experiment 

(LECP) 55-57 

Magnetometer (MAG) 53-55,57 

Photopolarimeter Subsystem (PPS) 50-51,57 

Plasma Science Subsystem (PLS) 55,57 

Planetary Radio Astronomy (PRA) 53-54,57 

Plasma Wave Subsystem (PWS) 53-54,57 


Radio Science Subsystem (RSS) 51-53,57 

Ultraviolet Subsystem (UVS) 48-50,57,153-154 

Science Investigation Support Team (SIS) 23-24,215 

Scientific Investigators 21,24,80,212-213 

Science Links 20,80-102 

Science Objectives 19-20 

Science Steering Group (SSG) 24,213 

Sequence Team 24,25,216 

Sequencing 2 

Sergeyevsky, Andrey 106-108 

Sigma Sagittarius (star) 94 

Smear 70,127-128,138 

Software 7,78-80 

Solar Flares 160 

Solar System 108,143,151,162 

Spacecraft Team (SCT) 24,216 

Speed (or Velocity) 52-53,64- 



Stars 50-51 



Test and Training 

Thruster Pulse Width Reduction 

TOPEX (space mission) 

Torque Margin Test (TMT) 


Trajectory Correction Maneuver (TCM) 

TCM B18 

TCM B19 

TCM B20 


Ulysses (space mission) 
Uranus System 












. 5,13,14-17,22, 

. 228-229 


506760 KM 


Atmosphere 188-190 

Internal Structure 187-188 

Magnetosphere 196-198 

Rings 141,190-192 

Satellites 4,192-196 

Usuda 27,79,125 

Venus 25,108 

Very Large Array (VLA) 27,79,125 

Voyager Project 

Mission Cost 135 

Project Events 245-252 

Voyager Interstellar Mission (VIM) 102,151-155 

Hypothetical Voyager-2 Diary 157-167 

Voyager Organization and Personnel 211-217 

Voyager Remembered 203-209 

Voyager Spacecraft 1,31-44,136-137 

Antenna, High Gain (HGA) 32 

Attitude & Articulation Control 

Subsystem (AACS) 34 

Attitude Control 138 

Axes 33 

Bus 32 

Canopus Star Tracker (CST) 34 

Computer Command Subsystem (CCS) 40-41 

Data Storage Subsystem (DSS) 38-39 

Digital Tape Recorder (DTR) 38-39,43,119-120, 


Failures 36,39-40,42,110- 


Flight Data Subsystem (FDS) 42,115-116 

Lifetime 141,154-155 

Power 36,38,138,155 

Phonograph Record 156 

Radioisotope Thermoelectric Generator (RTG) 36-38 

Reed-Solomon (RS) Data Encoder 126-127 

Receiver 39-40 


Scan Platform ... 
Actuators ... 



Sun Sensor (SS) 

35- 37,137-138 

36- 37,131 




Not fare well, 

But fare forward, voyagers . 

T. S. Eliot 


The time has come to say thanks. But where do we begin? Over 3000 
different people have contributed to the Voyager success, at one time or 
another, over the past nearly two decades. In fact, the number is even larger 
if we thank all of the science and news writers, and other media people as 
well, who thought enough of the Voyager story to share it with their reading 
and viewing audiences. As shown in Figure 3-4, several hundred folks are 
even helping out for the Neptune encounter, many of whom are named in 
Table 15-2. 

So, in terms of specific name credits, we’ll have to limit ourselves in this 
chapter to saying thanks to those who prepared the Guide. Originally the 
brainchild of Charley Kohlhase, a shorter version of the Guide was first 
issued for the Uranus Encounter. Because of its popularity, a decision was 
made to produce an expanded Guide for the Neptune encounter, the final 
planetary flyby for the epic Grand Tour mission. Within this Neptune Guide 
most of the raw writing and principal contributions for the various chapters 
were done by Jim Gerschultz, Robert Frampton, Charley Kohlhase, William 
Kosmann, Bob Neilson, Frank O’Donnell, Rex Ridenoure, Kate Robinett, 
and Anita Sohus. 

But writers alone do not a Guide make. It must be typed, reviewed, 
edited, and “laid out” for the printers. In these categories, special thanks 
are in order for Judith McGavin and Becky Harvey (for typing and moral 
support), Anita Sohus and Jeanne Collins (for coordinating), to Robin Dumas 
(for design, layout, and production), and Roy Halton, Lee Scot and Bruce 
Stout (for their graphics support), and Phil Gwinn (cartoons). 

Oh . . . the reviewers? Those having provided more than a handful of 
comments include Terry Adamski, Norm Haynes, Charley Kohlhase, Bob 
Mac Millin, Ellis Miner, Rex Ridenoure, and Edward Stone. 

There you have it. Once again, thanks to everyone who contributed to 
the Voyager success. To those who first conceived of the multiplanet Grand 
Tour opportunity; to those who convinced the Congress to fund such a long 
mission; to the Congress for seeing the wisdom in providing the needed 
resources; to those who designed the mission, spacecraft, and science; to 
those who improved the facilities at Earth Base as the Voyagers journeyed 


farther and farther from Earth; to the Flight Team who kept the Voyagers 
on course, developed the flight sequences, and carefully gathered the 
returning scientific data; to the many scientists who interpreted the exciting 
discoveries; to the media who publicized the results; and, finally, to all those 
loyal space fans who have cheered the Voyagers along the way as they leave 
the outer planets and head for the stars: Many thanks! 

Figure 21-1 . With its Grand Tour accomplished ’, Voyager 2 will join its companions 
Voyager 1, Pioneer 1 0 , and Pioneer Honan escape from our home star , the Sun. For 
thousands and thousands of years hence, each craft will traverse unabated through 
the vast expanse of space between local stars, orbiting around the core of our home 
galaxy, the Milky Way, shown here in an edge-on view compiled from months of 
Infrared Astronomical Satellite (IRAS) data. 




As Galileo could never have guessed how his discoveries 
would benefit mankind , neither can we fathom what impact 
the Voyager discoveries will have during the coming centu- 
ries. Our satisfaction comes from knowing that we were 
given a rare opportunity ; and that we seized it. 

Dave Field 

from the video Jupiter the Giant, Saturn the Gem 

Phil Neuhauser, Manager , JPL Public Education Office 

Handy Facts 

Voyager 2 Observations near Neptune 


Voyager's Past 


Two one-ton spacecraft were launched from Earth in 1977 to explore Jupiter and Saturn. Voyager 1 encountered both planets, using Jupiter’s gravity to go on 
to Saturn, and was flung by Saturn's gravity up out of the ecliptic plane in 1 980. Voyager 2 followed Voyager 1 to Jupiter and Saturn, and then went on to Uranus 
and Neptune, using the gravity of each previous planet to go on to the next one. This outer planets Grand Tour requires a planetary alignment that repeats 
only once every 1 76 years. 



• Turbulent colorful atmosphere 

• Narrow dusty ring 

• 9 active volcanoes on Io 

• Surfaces of all 4 Galilean moons 

• Lightning and aurora 

• 1000 mph winds 

• Elaborate, varied structure of rings 

• Surfaces of all major moons except Titan 

• Titan atmosphere 90% nitrogen 


• Tilted, off-center magnetic field 

• Rich terrain variety on Miranda 

• Surfaces of all 5 major moons 

• 10 new moons and 2 new rings 


Voyager 1 

• PPS failed (3/5/79) 

• PLS failed (11/23/80) 

• FDS memory B failed (10/6/81) 

• X-bandTWT failed (10/29/87) 

Voyager 2 

• Telemetry system degraded (9/23/77) 

• PPS, MAG, IRIS, CRS, PRA, PWS, LECP degraded (various) 

• Command receiver #1 failed (4/5/78) 

• Command receiver #2 degraded (4/6/78) 

• NA camera sensitivity degraded (8/1/81 ) 

• Scan platform AZ actuator stuck (8/26/81 ): later fixed 



Approach Date 

Closest Approach 
Altitude (km) 

Total Number of 
Images Returned 


Voyager 1 




Voyager 2 





Voyager 1 




Voyager 2 





Voyager 2 





(Voyager best imaging, resolutions in km) 

Voyager 1 

Voyager 2 











































Neptune and Triton 



Equatorial Radius (km) 



Mass (kg) 

1.023 xlO 26 

9.273 x 10 22 

Rotation Period (days) 



Orbital Period 
Mean Distance 

165 years 

5.88 days 

from Sun or Planet (km) 
Suspected Composition 

4.504 xlO 9 



Mainly H,He 

Mainly CH 4 , N 2 



Mainly CH 4 , N 2 


ch 4 , nh 3 

and H 2 0 


Key Events 

Enc. Rel.* 

Nereid-Best Imaging -11 h 20 m 

8/24 2:04 p.m. 

from Body 
Center (km) 


Inbound Ring Plane Crossing 

-57 m 

8/25 12:09 a.m. 


Neptune- Closest Approach 



1:06 a.m. 


Neptune-Earth Occultation Ingress 

+6 m 


1:12 a.m. 


Neptune— Earth Occultation Egress 

+55 m 


2:01 a.m. 


Outbound Ring Plane Crossing 

+1 h 30 m 


2:36 a.m. 


Start Triton Mapping 

+2 h 05 m 


3:11 a.m. 


Triton-Best Imaging 

+4 h 40 m 


5:46 a.m. 


Triton-Closest Approach 

+5 h 14 m 


6:20 a.m. 


Triton-Earth Occultation Ingress 

+5 h 43 m 


6:49 a.m. 


Triton-Earth Occultation Egress 

+5 h 47 m 


6:53 a.m. 


*Time relative to Neptune Closest Approach 
**Time signal is received at Earth, Pacific Daylight Time 

Voyager's Future 

Encounter Trajectory Plane View 

Voyager Interstellar Mission 

Voyager 1 and Voyager 2 will continue to sample the interplanetary/ 
interstellar media and solar wind, search for the location of the heliopause, 
and observe various ultraviolet sources among the stars. Power is expected 
to last until 2017, and propellant until 2035. Data will be sent back early 
in the mission at rates as high as 4800 bits per second, and late in the 
mission at 46-2/3 bits per second. Voyager 1 is heading towards the 
constellation Hercules, and Voyager 2 southwest of the constellation 



REV 1 1989 MAY 5